Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 202259
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
459 PM MDT TUE SEP 20 2016

.SHORT TERM...(Tonight through Wednesday night)
Issued at 409 PM MDT Tue Sep 20 2016

Moisture cast off by Tropical Depression Caine (downgraded as of 3
PM MDT) continued to spread northeastward over the area
throughout the day. This surge registered mainly as mid and high
level clouds which has kept temperatures down by 5 to 10 degrees
for all but northeast Utah and northwest Colorado. However,
deepening moisture over southwest Colorado ahead of a shortwave
trough moving over the Four Corners was sufficient to bring some
light rain to the San Juan mountains according to web cams.

Rain coverage will increase during the night as the shortwave
moves from the Four Corners across west-central Colorado during
the evening, then east of the Continental Divide by 12Z/Wed.
Areas south of the I-70 corridor are favored where precipitable
water will increase to between 0.80 and 1 inch as remnants of
Paine continue to infiltrate from the southwest. BUFR forecast
soundings for KDRO indicated a strong mid-level stable layer which
will inhibit thunderstorm development. As a result, expect
predominantly light to moderate stratiform precipitation overnight
and into Wednesday morning.

Lapse rates steepen and moisture deepens on Wednesday so expect
thunderstorms will begin to pop toward noon and beyond. The bulk
of the activity will remain in the south, but scattered activity
will spread into the Flat Tops and the Eastern Tavaputs and Roan
Plateaus. Expect storms to become more efficient rain producers
with locally heavy rain becoming likely. Storm strength decreases
Wednesday night while spreading into the northern zones.

Increased clouds and precipitation will continue the cooling trend
into midweek during the day while conversely moderating morning
lows.

.LONG TERM...(Thursday through Tuesday)
Issued at 409 PM MDT Tue Sep 20 2016

Thursday marks the beginning of a transitional phase into some
wintry weather that will impact the area Friday and Saturday.
Winds will pick up ahead of an approaching cold front driven by a
deep upper-level trough moving eastward across the Great Basin. At
this point, it does not appear winds will reach advisory criteria,
though, when combined with convective cells, could generate very
strong outflow winds. Precipitation coverage increases as the
front pushes eastward across eastern Utah during the night and
through western Colorado on Friday.

As the cold air sweeps in behind the front Friday night into
Saturday, snow levels will lower to near 7500 feet over northwest
Colorado and to around 8500 elsewhere. However, both GFS and ECMWF
indicated a lull in activity from late Friday afternoon into
Friday night which will limit mountain accumulations. Better
chances for accumulating snow arrive Saturday into Saturday
evening as the cold pool aloft sinks southward over the area.
Given snow levels around the 9000 foot level, it appears likely
there will be impacts due to accumulating snow across the
northern and central mountain passes and, to a lesser extent, the
southern passes in the San Juan Mountains.

Conditions improve Sunday and Monday, though moisture may return
Tuesday as the low begins to move northward after stalling and
possibly retrograding. Models diverge on the placement of the low
so confidence is low early next week

Temperatures will drop markedly Friday, possibly by as much as 15
degrees relative to Thursday. Freeze warnings are likely later
this week and may bring an end to the growing season for a number
of the higher valleys. Cool conditions will linger into the weekend
with a slow warmup beginning Sunday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 455 PM MDT Tue Sep 20 2016

Areas of stratiform rain will continue to increase in coverage
south of the Hwy 50 corridor spreading up to generally the I-70
corridor after 03z. Ceilings are expected to stay above ILS
Breakpoints, though mountain tops could be obscured at times in
heavy rain. KDRO and KTEX will have periods of rain, while other
TAF sites will see a 20-30 percent chance of rain. KVEL will
remain dry through Wednesday. Gusty southwest winds will mix to
the surface at times, creating some low level wind shear
elsewhere. These conditions continue through Wednesday with
thunderstorms possible after 12z.

&&

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

$$

SHORT TERM...NL
LONG TERM...NL
AVIATION...EH+Joe



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