Fire Weather Outlook Discussion
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FNUS21 KWNS 190709

Day 1 Fire Weather Outlook
NWS Storm Prediction Center Norman OK
0209 AM CDT Thu Oct 19 2017

Valid 191200Z - 201200Z


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

..Elliott.. 10/19/2017

...Please see for graphic product...

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