Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 171820
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1220 PM MDT Tue Apr 17 2018

.UPDATE...
Issued at 1219 PM MDT Tue Apr 17 2018

The winds have decreased behind the passage of the cold front late
this morning, thus the Wind Advisories for all zones but COZ014
have been expired. COZ014 will continue to see gusts up to 40 mph
through the late afternoon. Breezy conditions are expected to
continue across the region over the next few hours, but gusts
will remain below advisory criteria.

UPDATE Issued at 922 AM MDT Tue Apr 17 2018

The cold front has passed through those areas where Red Flag
Warnings are in effect. With the passage, humidities have
responded and increased above critical levels. While gusty winds
will continue, critical fire weather conditions will not be met.
With that, the red flag warnings will be cancelled shortly. Wind
advisories will continue as does the freeze warning. Will now take
a look at Thursday for the possibility of another Red Flag
Warning.

&&

.SHORT TERM...(Today through Wednesday)
Issued at 309 AM MDT Tue Apr 17 2018

Gusty southwest to west winds will persist over the area this
morning as we await the arrival of a strong cold front currently
nudging east across central Utah. Winds will gusts in excess of 50
mph in many areas today, even behind the frontal passage as
gradients remain exceptionally tight. A band of rain/snow will be
advancing through the area, generally north and east of a line
from Vernal, UT to Aspen by this afternoon, or immediately behind
the frontal passage. Out ahead of the front very dry conditions
remain in place and we have been seeing very poor relative
humidity recoveries this morning. There were signs earlier that
values would pull back above 15 percent, but those have diminished
as we continue to mix down drier air aloft. As a result, have re-
issued the Red Flag Warnings for several fire weather zones, see
the discussion below for more information.

The active weather conditions will continue right on into the
evening hours with sufficient instability to trigger a few
thunderstorms late in the day, generally over our central mountain
zones. Snowfall amounts have ticked up slightly with the latest
models targeting the Flat Tops with 3 to 6 inches through tonight.
Still sub-advisory and little impact in that particular zone.
Light amounts expected elsewhere, but combination of wind and snow
could cause some concern this evening over Vail Pass. Will allow
later shifts to evaluate that aspect of this forecast.

Finally, clearing skies and lighter winds will arrive over the
lower valleys of west-central Colorado and southeast Utah by
midnight setting the stage for another run below the freezing
mark. Have issued freeze warnings for these locations as we are
past median freeze dates and many sensitive plants/trees are
budding out at this time.

Wednesday will be a dry and cool day with a lot less wind
anticipated as high pressure builds overhead.

.LONG TERM...(Wednesday night through Monday)
Issued at 309 AM MDT Tue Apr 17 2018

Southerly flow will increase Wednesday night into Thursday with warm
air advection also taking place in this pre-frontal airmass ahead of
a closed upper level low pressure system moving into the Great
Basin.  H7 winds peak between 20 and 30 kts which is not as strong
as the previous few storm systems where H7 winds peaked around 50
kts. Even though the atmosphere will be very dry in the low to mid
levels Thursday, plenty of high level moisture exists so expecting a
thick high level cloud deck which would limit best mixing and
stronger winds from reaching the surface. Still a breezy and mild
afternoon though with enough wind in addition to the dry low
levels to potentially lead to fire weather concerns.

This closed low tracks across the Four Corners region Friday
afternoon. One concern with this system is that drier air tries to
work in Thursday night into Friday afternoon across the region,
wrapping in the center of the low. Specific humidity values are
between 1 and 2 g/kg which is not very high at all. Temps also
remain fairly warm with H7 temps ranging from +1C to -4C depending
on what model you want to believe when the best precipitation
potential exists. The better moisture wraps around this low over
the Front Range with good easterly upslope flow directed at the
eastern foothills. There is a TROWAL (TRough Of Warm air ALoft)
signature evident at H7 and H5 mostly wrapping over northern
Colorado into Wyoming and back through NW Colorado as the low
moves eastward across the region Friday evening. This would be
good if there was cold air in place, but there really is not.
Expecting high snow levels with this system with best
accumulations above 8000 feet, favoring the northern and central
mountains with snow. Hard to say at this time how much as the
amount of warming or cooling, amount of moisture, timing and track
all play a role in the final evolution. Our CWA really won`t see
the better accumulations until late Friday afternoon and evening
as the flow shifts around to a more favorable west to northwest
flow and the system is on its way out by Saturday morning. Models
are hinting at this system slowing down a bit as most closed lows
tend to do as we get closer to the event. So northern mountains
could continue into early Saturday afternoon before dry conditions
return for the remainder of the weekend as high pressure tries to
slide over the region through early next week. Some afternoon
convection is possible over the high terrain Sunday afternoon as
weak shortwave tries to invade the ridge but temperatures look to
return to above normal levels by early next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1219 PM MDT Tue Apr 17 2018

Breezy westerly winds up to 40kts will continue through the
afternoon in the wake of the cold front. The cold front is
currently situated across southwestern Colorado and will be
completely out of the region by later this afternoon. The
scattered showers are a bit more widespread across northern and
central Colorado currently, but will be moving southeast over the
next few hours. Winds will be decreasing and becoming mostly
terrain driven, and skies clearing through the overnight hours.
VFR conditions are expected on Wednesday as high pressure builds
in from the west.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Freeze Warning from midnight tonight to 9 AM MDT Wednesday
     for COZ006.

     Wind Advisory until 9 PM MDT this evening for COZ014.

UT...Freeze Warning from midnight tonight to 9 AM MDT Wednesday
     for UTZ022-027-029.

&&

$$

UPDATE...JPF/MMS
SHORT TERM...JDC
LONG TERM...MDA
AVIATION...JPF/MMS


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