Storm Prediction Center Day 1 Fire Weather Outlook
Created: Thu Oct 19 17:01:02 UTC 2017 ( | )
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ZCZC SPCFWDDY1 ALL FNUS21 KWNS 191659 Day 1 Fire Weather Outlook NWS Storm Prediction Center Norman OK 1159 AM CDT Thu Oct 19 2017 Valid 191700Z - 201200Z ...CRITICAL FIRE WEATHER AREA FOR PORTIONS OF EASTERN WYOMING... ...CRITICAL FIRE WEATHER AREA FOR PORTIONS OF CENTRAL MONTANA... A critical area has been introduced for portions of central MT. Nearly all short-term guidance is underdone with the strength of surface winds per 16Z observations, and it appears likely that sustained winds of 20-30 mph with higher gusts will continue through much of the afternoon in combination with lowered RH values of 15-20% and dry fuels to support a critical fire weather threat. No changes have been made to the surrounding elevated delineation encompassing a greater portion of central/eastern MT. Southerly winds of 15-20 mph are occurring across parts of central/southern MN within a return flow regime due to an enhanced surface pressure gradient between a surface low over Alberta Canada and high pressure centered over the eastern CONUS. Due to a prior frontal passage, substantial low-level moisture return is not expected to occur today, and RH values have already fallen generally into the 25-35% range as of 16Z. Further reduction in RH values into the 20-25% range seems likely, perhaps even lower than 20% on a brief/isolated basis. These meteorological conditions coupled with drying fine fuels owing to a relative lack of precipitation over the past week support the inclusion of an elevated fire weather area across parts of central/southern MN with this update. In the lee of the Sierras, gusty downslope winds may briefly overlap with lowered RH values late this morning into early afternoon across far eastern CA/western NV. However, latest fuel guidance suggests fuels are only marginally receptive across this region. The short duration of strong/gusty winds occurring with RH values of 15-20% and coupled with the marginal fuels suggests the overall fire weather threat remains too marginal to warrant an elevated area. No changes have been made to the elevated/critical areas across parts of central/eastern WY. See the previous discussion below for more information. ..Gleason.. 10/19/2017 .PREV DISCUSSION... /ISSUED 0209 AM CDT Thu Oct 19 2017/ ...Synopsis... Cyclonic flow will continue to overspread the western CONUS today, as a large-scale trough amplifies over the Pacific Northwest. As this occurs, mid-level flow is forecast to strengthen over the northern Rockies in response to the approaching mid-level trough. At the surface, low pressure is expected to modestly deepen while meandering over central/eastern Alberta. Meanwhile, an associated trailing cold front will shift eastward from the Pacific Northwest into the Great Basin. ...South-Central into Northeast Wyoming... Confidence in critical criteria being met and/or exceeded has increased enough for a Critical fire weather area to be introduced across northeast Wyoming. Southwesterly downslope pre-frontal winds are expected to strengthen by afternoon across the Elevated/Critical areas, as steep low-level lapse rates encourage efficient mixing to the surface of enhanced low-level flow. Furthermore, modest warming of a very dry low-level air mass will allow for minimum RH values to remain at or below 15% for several hours during the afternoon. While the degree of warming/drying should be relatively similar over the Elevated/Critical areas, the confidence in sustained wind speeds exceeding 20 mph is greatest over the Critical area. Whereas, sustained wind speeds of around 10-20 mph are expected over the Elevated area. ...Portions of Montana... The ongoing Elevated area was maintained, with only minor changes to incorporate the latest ensemble/deterministic guidance. By afternoon, the approach of stronger low/mid-level flow, associated with the aforementioned trough, coupled with relatively steep low-level lapse rates is expected to promote the development of sustained surface winds of around 15-25 mph. Additionally, warming of a dry air mass will allow minimum RH values to bottom out near 15-25% in an area of receptive fuels (owing in part to the lack of recent precipitation and ongoing drought conditions). The primary limiting factor for an upgrade to Critical remains the uncertainty in whether critically reduced RH values will overlap with sufficiently strong sustained wind speeds for a long enough duration. If trends in guidance suggest these conditions will overlap on more than a brief/spotty basis, then a Critical fire weather area may be needed in a future update for portions of the area. ...Please see www.spc.noaa.gov/fire for graphic product...
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