Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 202335

Area Forecast Discussion
National Weather Service Grand Junction CO
535 PM MDT Tue Oct 20 2020

.SHORT TERM...(Tonight through Wednesday night)
Issued at 255 PM MDT Tue Oct 20 2020

Pesky high clouds once again persist across northeast Utah and
northwest Colorado, which is having an impact on many elements the
models are not resolving very well. Hence, lower daytime
temperatures, a bit higher relative humidity and less mixing
resulting in less wind. This is enough to keep elevated fire
weather conditions at a minimum and very localized if that.
Thinking these high clouds are due to the presence of a strong
upper level jet that is laying across Wyoming. Elevated specific
humidity levels are present across Wyoming nudging into northern
Utah and Colorado, so this may be the reason for the persistent
high cloud cover. Regardless, this jet dips southward through
Idaho and Wyoming and edges closer to the northern Colorado border
Wednesday, resulting in a bit breezier conditions. Although like
previously mentioned, the high cloud cover could limit best mixing
potential to limit elevated fire weather concerns. Some areas look
to hit Red Flag criteria, but coverage and duration appears
marginal at best and not enough to warrant issuance of any fire
weather highlights. Temperatures for most areas will continue to
be above normal with dry conditions. Changes are on the way
towards the weekend, so enjoy it while it lasts. More details in
the long term section below.

.LONG TERM...(Thursday through Tuesday)
Issued at 255 PM MDT Tue Oct 20 2020

We start out in quasi-zonal flow Thursday through Friday night. A
trough axis will skirt northern portions of our forecast area on
Thursday allowing for some gusty winds across central Colorado
thanks to the continued tightened gradient. Additionally, extreme
northeastern portions of the forecast area bordering south-
central Wyoming could see some limited terrain-enhanced
precipitation as this wave moves through...but best moisture and
forcing will favor latitudes north of Colorado.

What`s going to happen as we move into the weekend and into early
next week seems to be the tricky portion of the forecast.  Keep in
mind that we`re still several days out, so things are gonna
change. But long story short, a couple sources of energy are
expected to impact the mid/upper levels allowing for a positively
tilted trough to move southeast from the Pacific NW toward our
region on Saturday. Model solutions seem to be in pretty good
agreement up to this point. Once that trough begins to dive
southeast toward us, that`s where the deterministic solutions
diverge. By Sunday night, the ECMWF has a closed low setting up
over the southwest CONUS while the GFS carries the system into the
Central Plains before slowing down. This difference is likely
related to the strength of the ridge and associated rex block low
off the west CONUS coast. Due to the complex nature of dynamic
interactions between these features, don`t have a strong feeling
as to which solution seems to be the best at this point. However,
the NBM output seems to be hedging bets in the direction of the
most recent ECMWF run. Looking at similarities and differences
across the global ensemble members, that seems to be the
reasonable guess.

With all of that uncertainty in mind, the current thinking is that
precipitation will start Saturday over the northern/central Colorado
mountains.  This lines up fairly well with the timing of the cold
front. As typically occurs in these parts, the cold front looks to
hang up overnight on Saturday across northern portions of the CWA.
By Sunday, this front moves south across the remainder of the CWA
and precipitation spreads across a good portion of the forecast area
(excluding some of southeast UT). The big question mark is how
will that precipitation continue across central and southern
portions of the forecast area on Monday. The most recent GFS run
rushes most precipitation out Monday morning while the ECMWF keeps
pops increased thanks to it`s cutoff low pattern. So, while the
model QPF seems to be a little high (as is usually the case),
certainty on the northern area QPF is certainly higher than the
central areas, with lowest confidence in the southern areas.

Temperatures take a roller coaster ride through the long term
periods. We start out hovering near or just above normals through
Saturday. As the aformentioned cold front moves through
Saturday/Sunday, temperatures drop about 10-20 degrees. Thus, a
pretty widespread freeze looks to be in order for the entire
forecast area on Sunday and/or Monday nights.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 530 PM MDT Tue Oct 20 2020

VFR conditions will prevail across all TAF sites for the next 24
hours, with passing high level clouds. Breezy conditions this
afternoon will diminish shortly after sunset, transitioning to
terrain driven flow overnight. Expect similar conditions Wednesday
(i.e. passing high clouds with gusty afternoon winds). However,
will see an uptick in winds, especially across northern TAF
sites, due to a disturbance approaching from the north.




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