September 24, 2018 – Red Flag Warning in effect until 5 PM Tuesday. Warming into Wednesday then cooling Thursday into the weekend. Areas of northerly wind today into Tuesday increasing fire weather concerns.
Upper troughing has shifted into the Great Basin as upstream ridging in the EPAC builds inland. N-S surface pressure gradient is around 9 mb between KCIC-KMFR and KDAX/KBBX VAD profilers showing about 20-25 kts of northerly wind 1-2k feet above the surface. Wind is trending up at some of the northern observation points attm.
Expect breezy conditions to become more widespread during the morning hours as momentum aloft begins to mix down. North wind of 10 to 15 mph with local gusts to 25 mph can be expected in northern and western portions of the Sacramento Valley, western Shasta mountains, and east slopes of the Coastal Range.
AMS warms today under increased subsidence and downslope flow. Afternoon max temperatures forecast to be about 3 to 6 degrees above normal. Warming temperatures with downslope wind will result in lowering humidity and increased fire weather concern. 1000-500 mb thicknesses continue to rise into midweek.
Max temperatures increase another 2 to 5 degrees Tuesday with a few degrees of additional warming the following day. Wednesday appears to be the hottest day of the week. Triple digit heat is possible in portions of the Northern Sacramento Valley Wednesday, with mid to upper 90s elsewhere in the Central Valley.
Some breezy northerly wind continues Tuesday morning, then pressure gradient decreases. A degree or two of cooling possible Thursday across portions of the CWA as heights begin to lower in response to deep closed upper low approaching off the CA coast. Forecast high temperatures, however, continue about 10 degrees above normal.
Extended Discussion (Friday through Monday)
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A significant cooling trend begins on Friday as models continue to show the Pacific low portion of the offshore Rex block pushing on to the coast. Winds should see a transition to more onshore which should aid in bringing a drop in temperatures by several degrees. If current models hold, the low center should be moving on to the norther coast during the day on Saturday.
Models continue to show this scenario so have introduced some precipitation threat over the northern mountains and the coast range Saturday afternoon. Models do show some difference in the details but confidence in some northern forecast area precipitation in the extended is growing.
More significant cooling is likely for Saturday with daytime highs dropping to from 5 to 10 degrees below normal. Still a bit more cooling will be possible on Sunday as the upper low opens and shifts east of the state. Extended models depart a bit more by the end of the extended Monday. ECMWF swings another low through norcal for a precip threat over the north third of the CWA while the GFS keeps the forecast area dry with the trough remaining offshore. Either way, daytime temps remain below normal.