July 12, 2014

Why western Washington will not face extreme heat this week. Plus major thunderstorm risk.

Although there has been a lot of talk about the regional heat wave, one thing needs to be made clear.

The western side of the Cascades will not experience extreme heat, while eastern Washington is burning.


And we have a very real threat of thunderstorms on Sunday, thunderstorms that could greatly complicate fire operations in eastern Washington, withe the potential for new lightning-initiated fires.

The visible satellite image this morning tells you a lot (see image).  Low clouds over the eastern Pacific are are still pushed against the coastal mountains.  That implies there is no offshore (easterly flow) and there is an onshore pressure difference (or gradient), with higher pressure offshore than over land.

Northwest meteorologists look at pressure gradients very carefully, particularly the onshore/offshore pressure gradient.  If higher pressure is offshore, marine air tends to move inland and keeps us mild. Offshore pressure gradients (higher pressure inland) minimize the marine influence and we can get very warm.

So let's look at the pressure gradients! (see below).  The time is in UTC/GMT (18 is 11 AM, 19 is noon PDT), SEA is the Seattle-Tacoma AP, OLY is Olympia, HQM is Hoquiam, OTH is North Bend, Oregon, YKM is Yakima, BLI is Bellingham, etc).  HQM-SEA and UIL-BLI are positive;  pressure is higher on the coast.  And the pressure is lower in Eastern WA (SEA-YKM is positive)

To get REALLY warm around here, we need the pressure gradients to reverse, producing easterly flow.

The forecast sea level pressures, surface winds, and lower atmosphere temperatures for 5 PM provide a different view of all this (see graphic).  You can see high pressure offshore and lower pressure in eastern Washington.  Superhot air east of the Cascade crest.  Western WA and Oregon will be much warmer than normal, but not extreme (mid to upper 90s).

Same general pattern of tomorrow, except much warmer in eastern Washington.  Bad for fires.

But something significant will happen tomorrow.   First, the air aloft will become increasingly unstable, meaning a little lift and there could be a release convection and thunderstorms.  A measure of the potential for instability is CAPE, Convective Available Potential Energy.  Here is the forecast CAPE for tomorrow at 2 PM.  For the NW, there are some fairly high values, particularly over the mountains.

Instability usually needs something to set it off, and that is generally a disturbance bringing upward motion.  And we have such a disturbance coming in tomorrow...an upper level trough (see image of 500 hPa heights at 5 PM tomorrow)
Our latest forecast model runs are producing thunderstorms and rain, particularly over the southern part of Washington, northern Oregon, and the Cascades.   Here is the 3 hr precipitation ending 5 PM Sunday and the 24 precipitation ending 5 AM Monday.   The thunderstorms could be shifted in time and space, but the atmosphere will be primed for some convection action tomorrow afternoon.

My big worry is for the eastern WA fires.   Strong winds from the thunderstorms could rev up the current fires and lightning from the storms could initiate new ones.  The upper level disturbance could initiate a weak onshore push of marine air, part of which could surge over the Cascades producing gusty winds. The fire fighters in eastern WA need to be prepared for changing conditions tomorrow.  And they need to very carefully watch the evolving weather situation and particularly thunderstorms and low-level winds.

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