Storm Prediction Center Day 1 Fire Weather Outlook
Created: Thu Dec 7 16:11:03 UTC 2017 ( | )
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ZCZC SPCFWDDY1 ALL FNUS21 KWNS 071610 Day 1 Fire Weather Outlook NWS Storm Prediction Center Norman OK 1010 AM CST Thu Dec 07 2017 Valid 071700Z - 081200Z ...EXTREMELY CRITICAL FIRE WEATHER AREA FOR PORTIONS OF SOUTHERN CALIFORNIA... ...CRITICAL FIRE WEATHER AREA FOR PORTIONS OF SOUTHERN CALIFORNIA... ...CRITICAL FIRE WEATHER AREA FOR PORTIONS OF SOUTHERN ARIZONA... The ongoing forecast is on track with dangerous and extremely critical fire weather conditions remaining likely especially in portions of southern California. Gusty easterly/northeasterly winds will increase to 30+ mph (with gusts potentially exceeding 60 mph in terrain-favored areas and higher elevations). Very dry surface conditions will also develop by afternoon, with RH values of 3-15% and very dry fuels supporting rapid spread of fires today. RH values will remain very low overnight (8-15%), with dangerous fire weather conditions continuing through early Friday. Another area of critical fire weather conditions will exist across portions of southern Arizona, where 20-25 mph northeasterly surface flow will combine with 7-15% RH values during peak heating hours. Another area of elevated fire weather conditions will develop in portions of the Lower Colorado River Valley, where northerly flow will approach 20 mph during the afternoon amidst very low (9-15%) RH. For additional information, see the previous forecast below. ..Cook.. 12/07/2017 .PREV DISCUSSION... /ISSUED 0142 AM CST Thu Dec 07 2017/ ...Synopsis... Dangerous and extremely critical fire weather conditions are expected across a broad portion of southern CA today. Within large-scale upper troughing over much of the central/eastern CONUS, an embedded positively tilted upper trough will continue moving south-southeastward over the Rockies/Plains early this morning. Enhanced north/northeasterly low- and mid-level flow around 30-40 kt on the western periphery of this upper trough has overspread the Southwest and southern CA, and these enhanced winds aloft will continue through at least the afternoon hours. Highly amplified upper ridging will remain over the eastern Pacific and parts of the West Coast. At the surface, a broad area of high pressure over the northern Rockies and Great Basin is in the process of restrengthening early this morning. A corresponding increase in offshore winds will occur across much of southern CA through this evening, supporting high-end critical to extremely critical fire weather conditions. ...Portions of Southern CA... The LAX-TPH pressure difference has strengthened to around -14 mb over the past several hours. Latest surface observations across southern CA show multiple locations in the mountains/foothills of Ventura and Los Angeles counties with sustained northerly to northeasterly winds of 30-35 mph, and gusts to 50-60 mph. RH values remain critically lowered in the 7-15% range for many areas. Given the strengthening surface pressure gradient plus the enhanced low- and mid-level flow mentioned above, sustained northeasterly to easterly winds of 30-40 mph will occur across much of the higher terrain of southern CA this morning through at least the evening hours. This includes the mountains/foothills and adjacent areas of far southeastern Santa Barbara, Ventura, Los Angeles, Orange, southwestern San Bernardino, western Riverside, and central/eastern San Diego counties. Stronger gusts of 50-80 mph will likely occur in favored higher terrain and passes where flow channeling will be maximized. RH values will easily fall/remain in the 5-10% range this afternoon/evening due to diurnal heating and low-level downslope warming/drying effects. Coupled with very dry/receptive fuels, these expected meteorological conditions support the continuation of an extremely critical fire weather area with mainly small changes. Large fire spread and extreme fire behavior will occur with any new/ongoing fires. Surrounding the extremely critical designation, critical fire weather conditions are anticipated across a broader portion of southern CA. Here, sustained winds of 20-30 mph, with gusts to 40-50 mph, and RH values of 5-15% will be common. Both downtown Los Angeles and the LA basin along with San Diego have been included in a critical delineation. These dangerous fire weather conditions are forecast to continue through the entire Day 1/Thursday period, although the surface pressure gradient will likely relax slightly late tonight into Friday morning. This should act to lessen winds somewhat across southern CA late in the period. ...Portions of Southern AZ... A critical area has been included for portions of southern AZ where sustained winds of 20-30 mph will overlap with RH values lowered into the 5-15% range for a few hours this afternoon. Latest fuel guidance indicates fine fuels are now receptive to new fire starts. These critical fire weather conditions are expected to quickly diminish late this evening with the loss of diurnal heating and corresponding RH recovery. ...Please see www.spc.noaa.gov/fire for graphic product...
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