Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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304
FXUS65 KSLC 220319
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
919 PM MDT Sat Apr 21 2018

.SYNOPSIS...High pressure aloft will maintain warm and dry
conditions across Utah through the day Sunday. A weak weather
disturbance will bring showers to northern Utah and southwest
Wyoming early in the upcoming week.

&&

.DISCUSSION...
Antecedent Conditions...
Extreme Drought is occurring from Provo south to Cedar City and
to the southeast. Severe Drought is occurring surrounding the
Extreme Drought, in areas south of Ogden and Duchesne to the
Arizona border. Drought conditions scale down to Abnormally Dry
along the Idaho and Wyoming border.

Big Picture...
During the past 5 days, the amplified pattern over the Pacific has
shifted toward being more zonal as the magnitude of the jet has
significantly decreased.

Water Vapor Satellite shows a deep closed low has progressed east
to the Oklahoma, with ridging extending from the southern Pacific
into the Great Basin. 400-250 MDCARS wind observations show a
northwesterly 110-140kt jet from southeast of the four corners
into northwest Texas.

Local Observations and Trends...
00Z KSLC RAOB shows a dry airmass with light flow. 700-500mb Lapse
rates are decently steep. Precipitable Water values range from
0.10"-0.20" mountains, to to 0.33"-0.55" most valleys.

A 1018mb surface anticyclone has centered across northwest Utah,
creating a 5mb northeasterly pressure gradient to far southwest
Utah.

24 hour trends include 10-20F warmer south of Salt Lake City, 5F
warmer across the north. Dewpoints depression have increased
(dried) 15-35F except 5-10F lower (moistened) across the
northwest.

Forecast...
Inherited forecast in great shape, with no reason to update this
evening. Beautiful weather under a ridge of high pressure today,
with even the valleys achieving strong warming compared to the
winter in this pattern, due to the higher sun angle this time of
year.

Return flow will bring an increase in mid level moisture
tomorrow, supporting diurnal instability from the Uinta mountains
south to Boulder Summit. With very steep 700-500mb lapse rates
across the region, isolated convection is likely in the
aforementioned areas.

&&

.PREVIOUS DISCUSSION...
The weak diffluent westerly flow aloft across the Great Basin
will maintain dry conditions and bring somewhat warmer temps to
the forecast area on Sunday. A shortwave moving into the Pacific
Northwest early Sunday will continue slowly east across the
northern Rockies/northern Great Basin late Sunday night through
early Tuesday.

The initial push of the mid-level trough into northwest Utah
should bring convection to northern Utah through Monday morning.
Modest, but fairly deep cold advection along with dynamic support
from the upper trough and adequate moisture should be sufficient
to generate scattered convective precip. Increase jet support
during the day Monday could lead to a thunderstorm or two late
Monday morning through the afternoon.

Both the GFS and ECMWF are showing the development of a mid-level
circulation center within the the trough across the northern
Rockies late Monday afternoon or evening. The result of this
development will lead to a cyclonic northwest flow across far
northern Utah/southwest Wyoming late Monday night through Tuesday.
Any precip will likely stay close to the colder air aloft which
will remain primarily over southwest Wyoming. Rapid warming aloft
Tuesday afternoon should bring an end to any lingering showers up
north during the afternoon.

Will have to keep an eye on possible isolated convection over the
higher terrain of central/southern Utah Tuesday. The global models
do show a narrow band of moisture concentrated along the old low-
level baroclinic zone stalled out over the southern half of the
state. Anticipating just isolated coverage of any convection that
develops, and then mainly over the higher terrain through early
evening.

An offshore low remains out to sea by mid-week with high pressure
ridging developing and continuing across the eastern Great Basin.
Dry and warming conditions will settle in and persist for the
forecast area of Utah and southwest Wyoming for Wednesday.

Models begin to differ in a solution for Thursday, with a brush-
by trough sweeping through Wyoming and down the Front Range of
Colorado as one model suggests. The cold air looks to remain well
to the east of our area, so kept very minor mention of any
precipitation confined to that area. Heights and upper level
temperatures still remain warm, so went with a warm bias on the
temperature forecast for Friday and Saturday.

Heading into the weekend, went with some slight chance mention in
the mountains and higher terrain of northeast Utah and the central
spine mountains as the upper level low begins an inland trajectory
with increasing southwest flow. Confidence is still somewhat low
with this feature, being the far extended time period and given the
lack on consistent solutions.

&&

.AVIATION...
Benign weather for the KSLC terminal during the valid TAF period.
An easterly pressure gradient should cause the typical southeast
drainage to back more easterly than normal.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

PUBLIC...10/Conger/Dewey
AVIATION...10

For more information from NOAA`s National Weather Service visit...
http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php



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