Area Forecast Discussion
Issued by NWS Grand Junction, CO

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586
FXUS65 KGJT 200932
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
332 AM MDT Mon May 20 2019

.SHORT TERM...(Today through Tuesday)
Issued at 332 AM MDT Mon May 20 2019

Deep and moist southerly flow will continue for much of the day
as our next upper level low slowly works east across the southern
Great Basin. The low is anticipated to reach the Four Corners
area by early this evening with bands of rain/mountain snow moving
from south to north throughout the day. Convective potential is
somewhat limited to get widespread thunder, but there will be a
few storms are with gusty outflow winds and small hail possible.
Snowfall will be heavy at times in the high country with the San
Juans favored today with a shift into the central and northern
mountains late this afternoon and tonight as the low approaches.
Snow levels will be down as low as 7k this morning, bounce up a
bit today, then drop again tonight with the heaviest accumulating
snow above 9k. Current highlights look good and now plans on
expanding coverage or upping amounts.

The upper low makes the jump across the Rockies tonight and
deepens as it drops into the plains. Flow aloft swings to the
north and northwest with moisture wrapping back around the low.
this will keep showers going through the day on Tuesday with
downshifting of the precip expected by late in the day. And then
we wait for the next system.

Temperatures will remain downright chilly for mid-May with
readings a good 15 to 20 degrees below normal for daytime highs.
Will need to monitor some of the high valleys that have vegetation
susceptible to freezing temps over the next few nights. At this
point, forecast values remain above 32 degrees for the next few
nights.

.LONG TERM...(Tuesday night through Sunday)
Issued at 332 AM MDT Mon May 20 2019

Tuesday evening looks to be in a transition period between systems
as the first low pressure trough that will have impacted the area
through Tuesday morning exits across the Dakotas and a second closed
low drops south into southern Nevada by Wednesday morning. This will
result in somewhat of a downturn in precipitation but still some
scattered showers around. Southerly winds increase Wednesday
afternoon for a breezy afternoon with storms developing over the
high terrain in the afternoon. The low will move through northern
Arizona before swinging through the Four Corners Wednesday evening,
leading to a noticeable increase in more moderate to heavy
precipitation as models are still indicating a band of precipitation
forming the western Colorado divide, bringing more snow to the high
country and rain to lower elevations. The placement of this
precipitation band is key in forecasting precipitation and high
elevation snowfall amounts as models are having a tough time with
it. The NAM is most favorable over the western Colorado divide, with
the GFS developing this band further west across the western
Colorado valleys before shifting east across the high terrain by
Thursday morning. A strong jet rounds the base of this trough and
allows this low to quickly eject into the northern High Plains by
Thursday evening. This keeps precipitation ongoing at least through
late Thursday afternoon. One concern with this southerly wave though
is the temperatures as they seem milder than the current system. H7
temperatures are in the 0C to +3C range across the western Colorado
mountains Wednesday evening through Thursday afternoon with this
moisture surge, putting snow levels up to around the 9000 ft level.
H7 temps cool to around -3C up north Thursday afternoon before this
system lifts northward and drier air puts an end to precipitation by
Thursday evening with WAA already beginning in southwest flow. With
the amount of liquid though, decent snow accumulations are possible
with best potential above 9000 to 10,000 feet.

A drier southwest flow develops Friday into the coming weekend with
another low eventually dropping south into the Great Basin by
Sunday. This low looks to open up and track across our area early
next week. However, the drier southwest flow will result in
significant warm air advection to bring temperatures back up towards
normal or slightly below.  This will feel warmer compared to the
cooler weather we have been and will experience. There is still
enough instability and weak waves that may result in isolated
convection over the high terrain each afternoon Friday through
Sunday but overall, most areas will be partly to mostly sunny and
much drier and milder.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 332 AM MDT Mon May 20 2019

Widespread rain and snow showers with isolated thunderstorms will
persist through Tuesday. Expect occasional CIGS at or below ILS
breakpoints with local MVFR/IFR conditions near showers. Mountains
will be frequently obscured over the next 24 hours. All TAF sites
will be impacted.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Winter Storm Warning from noon today to 6 PM MDT Tuesday for
     COZ004-009-010-012-013.

     Winter Storm Warning until 6 AM MDT Tuesday for COZ018-019.

UT...None.
&&

$$

SHORT TERM...JDC
LONG TERM...MDA
AVIATION...JDC



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