Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 252340

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
540 PM MDT Mon Sep 25 2017

.SHORT TERM...(Tonight through Tuesday night)
Issued at 257 PM MDT Mon Sep 25 2017

Longwave trough over central US will fragment through the period as
a shortwave ejects over the Northern Plains, and a secondary wave
cuts off over the southern Great Basin. Through the first 2
periods dry westerly flow will keep skies mostly clear and precip
chances very low. An isolated shower in the Uintah Basin is
possible this afternoon and evening. The main story for the short
term will be the chilly overnight lows. Freeze Warnings for
Northern and Southern valleys will go into effect at 9pm tonight.
Clear skies and calm winds should let the lows drop into the upper
20s to low 30s. Most areas saw their first freeze of the season
last night. We should remain a degree or two warmer on the lows
tonight but still below freezing in many spots. As the previously
mentioned cutoff low meanders around southern Utah Tuesday, flow
will switch to southerly bringing moisture back in across southern
Utah and Southern Colorado. Brought PoPs and sky cover up
slightly Tuesday afternoon and evening to account for the
increased moisture.

.LONG TERM...(Wednesday through Monday)
Issued at 257 PM MDT Mon Sep 25 2017

The cutoff low over the Great Basin will be the primary weather
maker from periods 3 through 7 as it slowly fills and drifts north
across Utah and far western Colorado. Moisture really builds in
Tuesday night and into Wednesday as models show the mean 700 mb
mixing ratios climbing to the 5 to near 6 g/kg range early
Wednesday. Given the adequate upper level support and high
moisture content, numerous showers and some thunderstorms are
expected across the southern portions of Colorado and Eastern Utah
Wednesday and Thursday. The NAM and GFS seem rather bullish with
well over three quarters of an inch of QPF in some spots late
Wednesday. Elected to cut down on the QPF, bringing it closer to
a half inch for now. One challenging aspect to the forecast is
nailing down the snow level. 700 mb temps are rather warm, with
values near 3 to 5 C over the southern San Juan Mountains in the
GFS. The NAM is much cooler, likely owing to enhanced convection
and dynamic cooling from the QPF bullseye. A backdoor cold front
should trap some warmer air over us keeping snow levels a bit
higher, and snow ratios a bit lower than than guidance originally
indicated. Right now, snow levels should hover around 10.5 to 11k
ft. Snow totals should be in the 3-6 inch range above 12k feet
into early Friday.

By Friday afternoon the GFS and ECMWF are in good agreement that
the cutoff low will be filling and moving into South Central
Wyoming. As the next trough moves down from the Pac NW, the cut
off should be absorbed into the flow amplifying west southwesterly
cyclonic flow over the region. Residual moisture and good jet
support should support at least isolated PoPs Saturday and
Sunday, mainly focused across the northern tier of the CWA. Monday
and Tuesday next week are a bit more uncertain as solutions
diverge significantly. ECMWF wants to bring a second trough in
from the NW. The GFS wants to keep dry and cool zonal flow in
across the area. Not a whole lot should be expected for POPs into
the long range given significant differences.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 540 PM MDT Mon Sep 25 2017

VFR conditions will prevail at all TAF sites over the next 24
hours with benign weather and diurnal winds. A closed low will
begin to develop to our southwest on Tuesday which could bring
some isolated to scattered convection over the San Juans after 18Z
as well as increased cloud cover further south.


CO...Freeze Warning from 9 PM this evening to 9 AM MDT Tuesday for



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