Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 212249
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
449 PM MDT WED SEP 21 2016

.SHORT TERM...(Tonight through Thursday night)
Issued at 347 PM MDT Wed Sep 21 2016

Southwest flow will persist as a strong low pressure system over
the Pacific Northwest begins to rotate toward the Great Basin.
This southwest flow will continue to bring moisture associated
with the remnants of Tropical Storm Paine over the region tonight,
keeping the chance of showers and a few late day/evening
thunderstorms. Some showers should continue to form through the
overnight hours. Nighttime low temperatures will be quite mild due
to clouds/moisture.

On Thursday the upper low pressure system will move into the
Great Basin with it`s associated cold front will reach into
western Utah by the afternoon. Ahead of these features a dry slot
will move over western Colorado area early in the day. However
sufficient moisture will linger to keep the chance of showers and
thunderstorms in the forecast, especially for the northwest and
far western areas. The flow aloft will become quite strong with
700 mb wind speeds up to 40 kts over eastern UT and far western
CO. One forecast challenge has been predicting surface winds
during the afternoon. Without pushing model guidance higher, wind
speeds remain below well high wind criteria. However with the
strong gradient flow any thunderstorms could produce very strong
gusts. On the down side however, clouds filling in during the
afternoon will inhibit the winds somewhat. So for now will not
hoist any wind advisories, but later shifts should continue to
monitor. Temperatures will remain on the warm side in the
prefrontal environment.

The stronger storms and most widespread precipitation will wait
for the evening as the front approaches. However the timing of the
front and trough passage are less than favorable for producing an
outbreak of strong thunderstorms. The surface front has been hard
to resolve as the deep low seems to overpower the windshift and
the flow remains southwest through Fri. But cooler air moves in
late Thu night as the south-north oriented upper level jet passes
from east to west over the area. The best model QPF is stretched
under the jet Thu evening and overnight. Snow is expected over the
mountains with snow possible over mountain passes during the late
night period, especially in the heavier convective showers. While
cooler than tonight`s, Thu night`s lows will remain somewhat
mild, especially in the valleys as the air mass remains unsettled.


.LONG TERM...(Friday through Wednesday)
Issued at 347 PM MDT Wed Sep 21 2016

The upper trough will approach on Fri with the main low center
ejecting to the northeast. The trough axis is progged to pass Fri
evening with a secondary lobe keeping energy over the area through
Sat as the low center slowly moves into the northern plains. There
continues to be some uncertainty in the timing details, but there
is good agreement that Fri will continue to be unsettled. The
north will be favored for precipitation, and much cooler
temperatures are expected area-wide. Behind the through the
northwest flow will keep showers ongoing over the favored northern
slopes through Sat. Snow levels could drop to around 8000 feet Fri
night...the lowest so far this season. Mountain passes may be
impacted.

The drying trend will continue on Sunday with a warming trend
beginning as high pressure builds to our west. While temperatures
will warm through next midweek...they will remain below normal for
late September. There will be little to no chance of precipitation
in this dry pattern.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday afternoon)
Issued at 449 PM MDT Wed Sep 21 2016

Moist south-southwesterly flow will continue to pump moisture
into the region. Light to moderate rainfall will favor the I-70
corridor and south with the best chances looking to be from the
San Juans south. VFR conditions will persist at all TAF sites but
brief periods of MVFR will be possible at times, especially in
stronger storms. North of I-70 will have more scattered coverage
with just VCTS favored this evening. More widespread coverage is
expected beginning tomorrow after 12Z as a strong cold front takes
aim at eastern Utah and western Colorado. Southwest winds will
strengthen with gusts 35-40 mph after 18z and any thunderstorms
that develop will produce even stronger gusts.


&&

.GJT Watches/Warnings/Advisories...
CO...None.
UT...None.
&&

$$

SHORT TERM...EH
LONG TERM...EH
AVIATION...EH


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