Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 151038

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
438 AM MDT Thu Mar 15 2018

.SHORT TERM...(Today through Friday)
Issued at 438 AM MDT Thu Mar 15 2018

Thunderstorms developed across northeast Utah yesterday evening
and became more isolated in coverage as they tracked across
northwest and west-central Colorado with a few lightning strikes
through the early morning hours. A couple lightning strikes are
still possible through the morning here and there so kept isolated
thunderstorms going through the morning. Temperatures remained
very mild in the moist and unstable southwest flow ahead of this
next cold front and upper level shortwave trough projected to move
through the area late this afternoon and evening, bringing colder
air with it. Showers will remain fairly scattered in coverage this
morning, before increasing this afternoon with the development of
thunderstorms as well. CAPE values will be around 200 to 400 J/kg
with steep lapse rates between 7 and 9 degrees C/km, which
supports the development of convection and thunderstorms.

Very tricky forecast today with this warm advective disturbance
impacting the area today through Friday morning. To start, due to
the milder air in the warm advective southwest flow this
afternoon, snow levels will start fairly high around 10,000 feet
MSL. H7 temps will range from -1C along the divide of western
Colorado to -4C in eastern Utah. As showers and thunderstorms
develop today, convection and convective banding will set up.
Where this convection occurs remains the question, which models
have a tough time handling and tend to over forecast. This is the
wild card. The colder air and front will not push through until
late this afternoon and evening, with H7 temps lowering to the -5C
to -7C range by evening. The flow remains primarily southwest
until late this evening and overnight when it switches to westerly
and the colder air moves in with H7 temps down to -7C to -9C range
across northwest Colorado by Friday morning. Moisture remains high
with specific humidity values in the 4 to 5 g/kg range this
morning, lowering to the 3 to 4 g/kg range this evening. Large
scale ascent increases across western Colorado from southwest to
northeast this afternoon and evening, increasing lift and chance
of precipitation and thunderstorm activity.

Snow levels will vary largely throughout this event due to
convection, which could drive snow levels lower at times. Expect
to see lulls from time to time as well due to scattered nature of
this convection, with precip becoming more widespread by late
afternoon and evening. Overall, thinking that 4 to 8 inches of
snow will fall on average across our western Colorado mountains
mainly above 9,000 feet with lesser amounts below this level.
However, due to convection, some areas could see more or less. The
lower valleys will see primarily rain with this event, with a
brief mix in possible late Thursday evening, but no more than a
dusting to few tenths possible. In coordination with the
surrounding Colorado offices, decided to hoist Winter Weather
Advisories for most western Colorado mountains starting at noon
today and continuing through 9 am Friday morning, and keep it
pretty broad-brush given the uncertainty with convection. Some
areas could see periods of heavy snow under convective bands,
while other areas could see very little if any snow. Evening
commute looks to be impacted as the front moves through, along
with blowing snow as winds will gust to 45 mph in the mountains,
so there is that as well which factored into the decision to hoist
advisories. Conditions will dry out late Friday morning as
precipitation comes to an end with lingering snow over the NW
Colorado mountains due to favorable orographics. Most the energy
will be taken eastward as lee side cyclogenesis takes place with a
sfc and H7 low developing over northeast Colorado. Shortwave
ridging will move overhead Friday afternoon, with southwest flow
returning ahead of the next system discussed below.

.LONG TERM...(Friday night through Wednesday)
Issued at 438 AM MDT Thu Mar 15 2018

Friday night, the 500 mb trough line will move east of the
Continental Divide, opening up eastern UT and western CO to
strengthening SW winds aloft ahead of the next wet Pacific system.
By Saturday morning, a broad 500 mb trough will cover much of the
Great Basin and Intermountain West. Its surface cold front will
be along the NV/UT border about noon and warm advection
precipitation ahead of the front will be spreading into eastern
Utah. The NAM12, GFS and ECMWF showing a standup front stretching
along the UT/CO border about midnight Saturday night. A well
defined precipitation band associated with this front and the 700
mb troughline will move across western CO Saturday night
throughout the day on Sunday. NW flow aloft and drying conditions
spread over the area by Sunday evening, with most precipitation
coming to an end Sunday night.

Monday through Wednesday, northwest flow will prevail with a weak
shortwave trough dropping through the area Monday night. Tuesday
and Wednesday should be dry under high pressure.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 438 AM MDT Thu Mar 15 2018

Scattered showers with isolated thunderstorms will continue
through the morning hours, with showers and thunderstorms
becoming more widespread this afternoon and evening. Expect
lowered CIGS/VSBY in showers with periods of heavy valley rain or
mountain snow, especially this evening and overnight. ILS
breakpoints will be reached at times where heavier showers occur
with conditions at valley TAF sites primarily above ILS
breakpoints. Showers will linger over the mountains through
Friday morning before conditions improve after 12Z Friday.


CO...Winter Weather Advisory from noon today to 9 AM MDT Friday for



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