Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 182140
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
340 PM MDT Thu May 18 2017

.SHORT TERM...(Tonight through Friday night)
Issued at 340 PM MDT Thu May 18 2017

The upper low responsible for significant snowfall across many
mountain areas and northern Colorado valleys is currently centered
somewhere over the Paradox Valley in southwest Colorado with the
overall circulation looking pretty elongated due to strong
dynamics on the northern side. These strong dynamics are resulting
in efficient moderate to heavy snowfall over the northern and
central mountains as well as the northwest Colorado valleys as
overrunning is overtaking any orographic influence. The upper
level flow on the northern side of the low is from the east, which
doesn`t tend to favor our side of the divide, however these
dynamics are strong enough to overcome that. Therefore, the
highlites for the northern and central Colorado mountains were
upgraded earlier this morning to Winter Storm Warnings for 10 to
20 inches of storm total snowfall, and looks on track. A TROWAL
(Trough of Warm Air Aloft) signature is very evident along the
Front Range of Colorado stretching westward across Wyoming. This
is nosing into northern Colorado as well, which is also allowing
cold air to remain trapped among the northern valleys of Craig,
Hayden and Steamboat Springs. This overrunning of the cold dome
allows for efficient snowfall production as well. So Winter Storm
Warnings remain in place for these northern valleys until midnight
tonight. It should be noted that the intensity should lessen this
evening as the upper low shifts further eastward, pushing the
better dynamics and nose of the TROWAL to the east. Hence, the
expiration time for the northern valleys. Areas south of the upper
low on the other hand, which includes the Uncompahgre Plateau and
San Juans, will see more of a scattered convective nature to the
precipitation with periods of moderate to heavy snow over the
mountains and lesser additional snowfall expected. These areas
remain in Winter Weather Advisories through Friday, which are on
track.

Regarding freeze potential for the valleys, those that will be
susceptible to those impacts to vegetation look to clear out
somewhat with less cloud cover. This includes southwest Colorado
valleys of Cortez area, Paradox Valley and the Montrose area.
Latest guidance indicates low temperatures tonight will be in the
upper 20s to low 30s for these zones, so upgraded the Freeze Watch
to a Freeze Warning. Looking at other sites, like the Grand
Valley, cloud cover will remain in place with downsloping drainage
winds keeping the atmosphere well mixed overnight, therefore temps
look to remain above freezing once again.

As the upper low shifts eastward, it is projected to be over the
central divide mountains by Friday morning, with moisture
rotating around the northern portion. Precipitation intensity will
lessen throughout the day on Friday with the northern and central
Colorado mountains being favored in northwest flow. The NAM is
pretty spotty with showers, showing a largely convective nature to
the precipitation, while the GFS and ECMWF hang onto the showers a
bit longer into Friday afternoon. Largely improving conditions
will be seen from the west with drier air moving in due to a
building ridge of high pressure over the west coast. Thinking
periods of moderate to heavy snow still remain possible over these
mountain areas with lulls from time to time. The current end time
of 6 pm Friday for the winter highlites still looks reasonable to
account for this discrepancy, as these lows are sometimes slower
to move out. Some valley fog may develop both tonight and Friday
night due to recent precipitation.

.LONG TERM...(Saturday through Thursday)
Issued at 340 PM MDT Thu May 18 2017

Saturday overall looks like a drier day as the upper low moves
across the northern high Plains. Some showers may linger over the
northwest Colorado mountains due to continued orographics but
intensity looks fairly light. This upper low will move into
Ontario and the northern Great Lakes by Sunday with a few
shortwave disturbances rotating around the periphery, clipping our
northern mountains for a chance of some convective showers
developing each afternoon from Sunday through Tuesday. The ridge
of high pressure will build over the Pacific Northwest by Tuesday
night into Wednesday and slide over the CWA through Thursday,
leading to mostly drier conditions. Still kept some isolated
shower activity over the mountains due to uncertainty in the
models that far out in when it moves the ridge overhead.
Regardless, temperatures will rebound and warm back up to normal
levels by Sunday and continue towards above normal by mid-week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1108 AM MDT Thu May 18 2017

Aviation conditions will remain poor through the afternoon. Rain
in the lower elevations with mountain snows above 7000 feet will
result in ceiling heights in the MVFR to occasionally IFR range.
Visibility will be reduced in periods of heavier precipitation to
IFR levels. Mountains will be obscured especially in the higher
terrain to the east where heavier snow will continue. Expect a
general improvement in conditions later this evening and overnight
with precipitation decreasing in intensity overall across the
region.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Winter Weather Advisory until 6 PM MDT Friday for COZ003-017>019.

     Winter Storm Warning until 6 PM MDT Friday for COZ004-009-010-
     012-013.

     Freeze Warning from midnight tonight to 10 AM MDT Friday for
     COZ011-020-021.

     Winter Storm Warning until midnight MDT tonight for COZ002-005.

UT...Winter Weather Advisory until 6 PM MDT Friday for UTZ023.

&&

$$

SHORT TERM...MDA
LONG TERM...MDA
AVIATION...MAC



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