The Real Story of the D-Day Weather Forecast

The anniversary of D-Day is coming this week, and a movie has just been released about the D-Day weather forecast (called Pressure).

The movie follows the plot of the play of the same name, describing how a weather forecaster (James Stagg) pushed to delay the landing due to a storm, was opposed by an egotistical American meteorologist, and that the German forecasters were wrong, leaving the German defenders unprepared.   

New information from Allied and German archives and modern reconstructions of the weather during the landing period reveal that some of the "Pressure" story is not correct.

June 6, 1944

The landing in Normandy had several key environmental requirements.   

A full moon was needed to provide sufficient illumination for gliders and parachute troops the prior evening. No low cloud base that would prevent close air support and accurate naval bombardment along the coast.    Low tide conditions, so dangerous beach obstacles could be avoided.   Furthermore, winds had to be light so that waves and swell would be minimal.

June 4-7th had the requisite lunar and tide conditions, with the next opportunity not until later in the month.  And it was a matter of time before the Germans figured out where the real invasion would take place.  There was strong pressure to move sooner rather than later.

Three forecast groups were represented in the overall meteorological team: two British (UK Meteorological Office and British Navy) and one American (headed by Dr. Irving Crick).  The meteorological leader was the Royal Air Force's James Stagg.    

The British meteorologists made use of traditional forecasting approaches that relied on the Norwegian Cyclone Model, with a typical evolution of cyclones and fronts (see below), while the Americans used a controversial "analog" approach, where one searched for similar weather situations in the past.

Weather prediction was a primitive, subjective affair at that time.  There were no numerical weather prediction models, little understanding of jet streams and upper-level weather features, and an acute lack of observations, particularly over the ocean.

The situation on June 4, 1944, was problematic (see the Allies' sea level pressure analysis below).  A fairly strong low center was approaching northern Scotland.  The lines are isobars, lines of constant pressure, and where they are close together, the winds would be too strong.  Plus, a cold front was approaching (the line with the triangles on it).   

Not good for invasion:  winds would be too strong on the beaches, with lots of wave action and high water level (could not see the obstacles).

The weather map on June 5th indicated that the low had strengthened but had moved to the northeast (see below).  An area of high pressure centered west of Spain was starting to extend northward into the region.  Winds would still be significant, but the situation might be improving.

The forecast team under Stagg predicted the low would continue northward, producing conditions just good enough for D-Day on June 6th.   The actual weather map for June 6th (below) suggests that Stagg and associates made the right forecast for the wrong reason. 

As noted in an excellent recent paper by Anders Persson, the storm did NOT move to the northeast as predicted but weakened rapidly and slid southeastward.  Problematically, pressure differences over the channel were large enough to create significant wave action, which caused extensive sea sickness, high water levels on the beaches, and lost equipment and deaths.  

The disliked American forecaster  (Irving Krick) has been right about a pattern change that would push the low south rather than to the north, as predicted by Stagg and friends.

The situation was marginal, but just good enough for the landings in Normandy.

It has been thought that the Germans had poor weather maps over the eastern Atlantic because they had few weather assets over the Atlantic (and none over North America).  

But as noted in the article by Anders Persson, it appears the Germans had decoded Allied weather communications and possessed relatively good weather maps.  They assumed that the marginal conditions on June 6th were not good enough for an invasion, resulting in a lack of readiness and the absence of General Erwin Rommel to visit his wife on her birthday.

But the story does not end there.   

During the past decade, the meteorology of the period has been simulated using modern weather models and data assimilation tools, giving us a far more accurate description of what occurred during the D-Day period.

Below is an animation of the hourly precipitation (color shading) and sea-level pressure on June 6, 1944, made by the European Center (the ERA5 analysis).  You can see the low northeast of England weakening and moving southward.    Not what Stagg expected.

Although marginal, conditions were just good enough for a successful landing on Normandy, although the high winds were very problematic. 

Severe Thunderstorms with Strong Winds, Massive Lightning, Heavy Rain, and Hail Hit Eastern Washington

As predicted, a line of strong thunderstorms moved through eastern Washington yesterday, with dramatic impacts.   The radar image at 8:45 PM last night is shown below.  Red is associated with very heavy rain or hail.

The line of thunderstorms was associated with strong gusts from storm outflow.  The maximum gusts yesterday are shown below.  Some reached over 70 mph!

The result was lots of downed trees and branches, with thousands losing power.   As an example, here is a picture sent to me by Bob Powers of Mabton, Washington (near Yakima).

This event brought massive lightning, with thousands of individual strokes (see below), a measure of the great instability and vertical development of these storms.

This event also produced very heavy rainfall and impressive totals.  Many locations exceeded 1 inch (see below).

And some exceeded 2-3 inches!

This heavy rain not only moistened the soil, but will keep the reservoirs high.

A stunning aspect of this event is how well high-resolution weather models predicted it.  We have come a very, very long way during the past decades.

This weekend should bring mid-60s in the west and around 80F in the Columbia Basin, with dry conditions.  Enjoy.

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Announcement

I will hold a special online Zoom session at 10 AM on Saturday for Patreon supporters. 

Severe Thunderstorm Threat On Thursday

There is a significant chance of several thunderstorms tomorrow (Thursday) over eastern Washington, eastern Oregon, and northern Idaho, with the potential for heavy rain, lots of lightning, and strong winds.

The Spokane National Weather Service office has a severe thunderstorm warning out:

And the NOAA Storm Prediction Center has a moderate level of warning (yellow), which is very unusual for our region.

The predicted strong thunderstorms will move in during the afternoon.  The National Weather Service high-resolution HRRR model shows an intense line of thunderstorms (convection) moving through around 10 PM Thursday.

Three hours later, the line crosses the Canadian border with some showers reaching western Washington.

The line of thunderstorms will be associated with very strong winds in eastern Washington, with some gusts exceeding 60 mph (see wind gust forecast at PM tomorrow, below).  I suspect the wind farms will be "feathered" to reduce damage.

Heavy precipitation will also fall, with the HRRR model going for huge totals over the Oregon Cascades.  Substantial rainfall (over a half inch) over the Yakima River drainage.

The UW forecast model is also producing heavy rain over eastern Washington, which should be a great relief to those concerned about drought.

Localized flooding in river valleys is possible.

Why the potential for such an intense event?    

The "set-up" is illustrated in the upper-level map below (for 500 hPa pressure, about 18,000 ft).

An upper-level trough of low pressure is approaching (yellow arrow), producing uplift that can release atmospheric instability.  A second trough to the south (red arrow) is moving moist, unstable air in from the southwest northward into our region.   Upslope on the eastern slopes of the Cascades will contribute additional meteorological fuel.

In short, later tomorrow will be an exciting time from the Cascades and to the east.

Announcement

I will hold a special online Zoom session at 10 AM on Saturday for Patreon supporters. 

Substantial Precipitation Has Returned to the Pacific Northwest: What Has Changed?

The western U.S. weather pattern that has dominated most of the winter is collapsing and will not be returning.

With this change, substantial moisture will spread over the region, particularly the eastern side.

Enough precipitation that some of the exaggerated drought worries by some local media, amateur YouTube channels, and others should "evaporate."

The big change is evident on the latest visible satellite image, which shows a potent front making landfall on the Northwest 

There has been a lot of problematic information provided by some, so let me describe what has and probably will happen.

Overall Wet and Warm

The essential aspect of this past winter season for our region is that it has been wet and warm, with ample water to fill the reservoirs but sufficiently warm to reduce the snowpack by roughly 50%.

To demonstrate this, there is the precipitation data from October through April for Washington State from 1900 to 2025.   This year was a bit wetter than normal.

The plot for temperature is found below.  This year was considerably warmer than normal, although not a record (which goes to the crazy warm year of 2015).

Interestingly, our precipitation this year has been episodic, with very wet atmospheric rivers and extended dry intervals, something I will explain below.

So what has been going on?

This year, we had a very persistent upper-level (500 hPa, about 18,000 ft) pattern (see below).  The yellow/red areas indicate enhanced high pressure--also known as ridging.

Such a high-pressure area is associated with warm, dry conditions over the western U.S.   But since we were on the edge of it, sometimes an atmospheric river from the southwest or west could push in, giving us substantial precipitation (such as in December and March).  

Regional reservoir managers were very wise and saved the water from these short-period atmospheric rivers--that is why we are in decent shape right now.  They should all be given a gold star.

It is important to note that this persistent upper-level pattern has little to do with global warming, a claim in some media accounts. 

 I have read the literature on this issue and have published on this exact topic, so I am not speculating about it.  On the other hand, some of the warming (around 2F of it) may be ascribed to the slow warming of the planet due to increasing greenhouse gases.

The upper-level pattern is changing now (see below).  

Over the next five days, low pressure (blue colors) will dominate the West Coast from Alaska to Baja, with the "evil" high-pressure ridge (red colors) moving well to the east.

10-15 days from now is shown below.  

OMG.  A deep trough over our region.  That means wet.  

The lastest 48-h precipitation forecast for our region (below) shows substantial precipitation.  

Look closely....very substantial precipitation over the Yakima Basin!  The weather gods have heard us!  Or perhaps the weather gods want the Seattle Times to cool the hype. 

The Yakima water managers were very wise to reduce river flow this week, since their region will get a thorough wetting.

The rain is not going to stop.  The totals over the next 15 days are substantial (see below).  

Good news for agriculture, good news for weather supply, and good news for wildfires.

This precipitation will ensure that the Yakima reservoirs will stay full and should keep the westside reservoirs in good shape (they will fill, and there will be no need for irrigation in the west with rain falling).

The Weather Gods are Not Happy With the Weather Coverage 

of a Certain Newspaper

The Heaviest Precipitation of the Year Ahead in Some Locations?

If you ask most Washington State residents when the heaviest precipitation of the year generally occurs, they will tell you during the late fall and winter.  

It turns out that this is true for many, but not all, Washington State locations.    For some locations in eastern Washington, JUNE is the wettest month or at least equal to December.   

For example, consider Northport, a town in northeast Washington on the Columbia River (red marker on the map)

At this location, June has as much precipitation as December!  At some locations in northwest Washington and southeast BC, June is clearly the wettest month

How can this be?

One reason is that thunderstorms and convective showers are increasing rapidly this time of the year, with maximum thunderstorm activity in late June (see map below)

But there is another reason.....and we will experience this feature this week.

During the cool season, the westerly (from the west) jet stream is strong, and eastern Washington is in the rainshadow (see map of annual precipitation below)

But during May and June, the westerly jet stream weakens, and sometimes a trough of low pressure slides south of Washington, with moist southeasterly flow moving northward up its eastern side into eastern Washington.  No rainshadow!

This is exactly what is predicted to occur on Monday and Tuesday (see upper level map (500 hPa, about 18,000 ft) for early Tuesday).

Eastern (and western) Washington needs to be prepared for substantial spring rains.

The total through Wednesday morning is substantial in the west, and much of eastern Washington gets moistened.

Another low-pressure trough will move in later in the week, with the accumulated precipitation total through Sunday at 5 PM shown below.  Very, very substantial amounts in eastern Washington and Oregon.  Less irrigation will be needed next week.

 Enough precipitation over the Olympics and eastern Washington that I suspect the drought talk should fade.

 

A Wet Memorial Day

 If you are planning an outdoor excursion on Memorial Day (Monday), I have a suggestion:  bring an umbrella and wear a warm jacket.

Consider the latest forecasts.  The total precipitation through Sunday at 5 PM shows only a little drizzle on the windward side of some local terrain.

But on Monday, a strong upper-level trough will move in (see upper level map below, with blue indicating lower pressure than normal...troughing), producing much colder than normal temperatures (highs in th 50s) and substantial precipitation.

Here is the forecast precipitation total through Tuesday morning:  western Oregon and Washington will be thoroughly watered down, with some rain extended over the Cascades into Yakima and Kittitas counties. 

More precipitation is predicted for later in the week.  In fact, the regional totals through early June are scary wet (see below).  Perhaps this will comfort folks worried about drought. 😊

It is important to note that Memorial Day weekend is infamous for wet conditions in our region.  It illustrates this soggy fact, below is a plot of the historical precipitation at SeaTec from May 23 through May 28.

Most years have some precipitation over that period...and occasionally it gets quite wet (almost 2 inches).   This year, we will enjoy about a half inch over that period, so we will be a little wetter than normal.     That's fine...we can use it.

Why the Washington Drought Emergency Should Be Dropped Immediately

It is time to clear the air.   

To definitively correct misinformation being pushed by a Washington State agency, the Seattle Times Climate Times Climate Lab, and some amateur YouTube channels.

The claim that this summer represents a DROUGHT EMERGENCY for Washington State.   

It is simply and demonstrably false.

In a previous blog, I showed that our reservoirs are fuller than normal, soil moisture levels are good, our annual precipitation is at or above normal, and that current crops are doing well. 

But like some vampire, the drought claims can not be killed so easily.   

In particular, the media, YouTube amateurs,  the State Department of Ecology, and others are pinning their drought claims on the Yakima River drainage and this year's low snowpack.

Below, I will demonstrate why they are very much wrong. 

Yakima and Kittitas counties, through which the Yakima River flows, are relatively arid locations, being located in the rain shadow of the Cascade Mountains (see below).

Only with substantial irrigation can they provide the agricultural bounty for which they are well known.

This irrigation comes from two sources.  

Much of Yakima and Kittitas agriculture gets its water from the Yakima River, with some contributions from the Columbia on the eastern side of the area (see map below).   Everyone agrees that the Columbia will have plenty of water this year, so there is no suggestion of drought for the majority of the irrigated land in eastern Washington (blue, purple, and yellow colors below)

The Yakima River drains off the Cascades, starting near Snoqualmie Pass, and is also fed by a few tributaries, such as the Cle Elum, Teenaway, and Naches rivers.

These rivers are mainly fed by mountain rain and melting snow.  Since most of the precipitation falls in winter and spring, while most of the irrigation demand is during the dry summer (particularly July and August), water storage is required.

One source of storage is the Cascade snowpack, which does most of its melting in May and June.  Thus, snowmelt is only a short-period source of water... a fact that will be important later.

And then there are the Yakima River Basin reservoirs (see below), which are filled by both rainfall and snowmelt.

And finally, there is spring and summer rainfall, which is modest in this region.

To fully supply water for all agricultural and human needs of the Yakima irrigation district requires about 2.3 million acre feet for April through September.    The reservoirs can hold about 1.1 million acre-feet, or roughly half what is needed.

The remainder must come from the river flow, supplied by either mountain rain or snowmelt.   

A big issue is timing:  only the reservoirs can supply a large amount of water during mid-summer (July and August) when demand is greatest.  

Keep this fact in your mind...it is crucial.

Now, let's consider the water situation this year.   The annual precipitation was actually wetter than normal over and near the Cascades (see difference in normal during the water years from October 1 to  May 6, below).  So plenty of water falling from the sky.

But much of this precipitation was associated with warm atmospheric rivers and substantial amounts fell as rain, particularly at lower to middle elevations.   Particularly strong atmospheric river events occurred in December (which produced substantial flooding) and March.

Because of the warmth of those atmospheric rivers, the snowpack was below normal this season over the central and southern Cascades, with the Yakima Basin topping out at around 50% of normal (see below).  Not zero, not 20%.   This is going to be important as well.

The substantial precipitation and the wise actions of those running the Yakima reservoirs have led to the Yakima reservoirs filling well in advance of normal.   Filling early without the need for snowmelt.    

Check out the figure below, which shows the water storage this year (blue line) compared to normal (red line).  Stunning.  We are already above the normal mid-June peak!

The Yakima Reservoir storage will remain topped off for several more weeks.  Why?   There is still considerable snow left to melt at higher elevations in the Cascades during the next week or so (see below).

.And the current model forecasts indicate substantial precipitation over the region next week (see totals through next Saturday).

There will be plenty of precipitation and snowmelt to keep the Yakima Reservoir system topped off through the end of the month.  No drying heatwaves are forecast during this period.

With Yakima Reservoir full in early June, there is a near certainty of sufficient water for Yakima County agriculture this summer.

I am not speculating....I checked.   Full reservoirs starting the summer are associated with ample water for agriculture in Yakima and Kittitas Counties.  There have been no examples of serious agricultural water issues for years in which these reservoirs start off the summer full. 

So how could the Department of Ecology and others get this so wrong?

They did not consider several unusual aspects of the past year.   

We started off with very low river and reservoir levels last fall.  But then we had crazy heavy warm rain in December, which caused a massive increase in river levels on both sides of the Cascades.   To illustrate, the figure below shows the flow on the Yakima River at Umtanum over the past two years (see below, black line).  Then, in March, another major event occurred.  Both were at record levels (dark blue lines)

Knowing about the low snowpack, the Yakima system water managers saved a portion of the huge influx of water,  pushing storage WAY above normal.  Very wise.

Normally, some of the melting snowpack is used to fill the reservoirs, but because of the heavy rain, it was not needed.   Thus, the melting snow could be used simply to keep the reservoir topped off and support reasonable flows.   

Because of the above-average precipitation last winter and the full reservoirs, we did not need to have a normal snowpack this spring..... 50% could do.   And the substantial rain also provided soil moisture west of the Cascades.

Those falsely calling for a drought emergency love to note that reservoirs can only hold about half the needed water for Yakima irrigation, and thus, they suggest that we need 100% snowpack to get through the summer. 

False!

But they are thinking too simplistically.  During the spring and early there is still substantial precipitation and water draining into the system, providing water for irrigation and other uses.  And we never use 100% of the snowpack water, with substantial amounts running into the sea.   Don't believe me?  Here is the average flow at the Dalles on the Columbia River.

A snowpack with 50% of normal snow still has a LOT of water.

In summary, the Drought Emergency Declaration by Washington State should be dropped immediately.  

Washington State residents deserve accurate information.  Washington agriculture should not be undermined by ill-advised, unscientific declarations.