Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 280913

Area Forecast Discussion
National Weather Service Salt Lake City UT
313 AM MDT Wed Jun 28 2017

.SYNOPSIS...A westerly flow will remain over the forecast area
today. A cold front will cross northern and central Utah
late today and tonight before high pressure returns beginning


Water Vapor Satellite shows a baggy trough approaching from the
Pacific Northwest. 400-200mb MDCARS wind observations place a
60-90kt subtropical westerly jet from California through the
Central Rockies. A weaker cyclonic jet was off the coast of
British Columbia and Washington. GOES/SLC 00Z RAOB/HRRR indicate
that precipitable water values vary from 0.15"-0.25" mountains to
0.35"-0.60" valleys.

Warm and dry, with west southwest winds become breezy today with
a cold front traversing the state from northwest to southeast this
afternoon through tonight.

SREF indicates strong destabilization across the north and
central portion of the state. Gradual height falls occur as a
series of waves carve out the trough from the Pacific Northwest
into northern Utah. Expect isolated to scattered convection along
the Idaho border coincident with the left exit region of the
approaching cyclonic jet.

While cloud bases are around 500mb this is not a typical dry
thunderstorm day. First there are dynamics, including a good
chance of seeing 30 kts of effective shear. Second the instability
is pretty strong, so today will likely be a hybrid dry/wet
thunderstorm day. Storm motions are about 30kts from the west, and
with DCAPE between 1000-1500j/kg, would expect a few severe wind
reports near the Idaho border this afternoon and evening.

Sunny cooler and a touch more moist tomorrow behind the cold
front. Downslope areas of the eastern valleys will likely remain
breezy out of the northwest.

Ridging builds over the west coast early Friday, then slowly
shifts eastward through Friday and Saturday. This will produce a
warming trend across Utah and southwest Wyoming. The drier and
more stable airmass on Friday should lead to a quiet day
convection-wise, but this could increase slightly on Saturday
ahead of a shortwave trough.

Models continue to be relatively inconsistent about how to handle
this shortwave, even though it is now in the day 3/4 forecast. The
00Z GFS seems like a nice compromise between the solutions that have
been presented the past few nights, with part of the trough cutting
off and heading toward southern California, while the bulk of the
disturbance passes along the Utah/Idaho border as an open wave late
Saturday through Sunday. This wave could kick up winds a bit and
keep the overnight periods fairly warm, but otherwise there does not
appear to be a huge impact from this system, as it lacks both
moisture and significant cold advection.

After that wave passes, ridging likely re-builds over the Rockies
for the beginning of the week, leading to more warm and dry weather
through at least Independence Day.


Winds at the SLC terminal are expected to switch to the
northwest around 18-19Z, though there is a slight chance that this
switch could occur as late as 21Z. There is also a 10 percent chance
that the terminal will be affected by showers and thunderstorms
forming to the north this afternoon/evening, with gusty erratic
winds as the most likely impact.


ERC values have risen to between the 80th and 96th percentile
across southern and central Utah, with a few reporting stations
above their 97th percentile. Far northeast Utah remains below
50th percentile, but the remainder of northern Utah is climbing
above the 50th percentile.

Multiple concerns regarding fire weather. The Haines index will
be a 6 today across central Utah.

Continued bone dry conditions and warm with a cold front forecast
to press through the state from northwest to southeast this
afternoon through tonight. This will lead to strong gusty westerly
winds when combined with widespread single digit RH will support
critical fire weather conditions.

This afternoon and evening we are expecting isolated to scattered
thunderstorms along the Idaho border. They will have some
moisture with them, but also pose the threat of lightning and
gusty/erratic winds due to microbursts.

Cooling and a little moistening tomorrow behind the cold front.
Still northwest winds will remain gusty in the eastern valleys and
it should be dry enough to continue to support critical fire
weather conditions in these areas.

A ridge gradually builds in for the weekend, increasing Haines
Index to 6 again, along with a warming and drying trend. There
could be some isolated showers and thunderstorms in the mountains
each afternoon and evening from Saturday through Monday.


UT...Red Flag Warning until 10 PM MDT this evening for UTZ479-482-484-

     Red Flag Warning from 10 AM this morning to 10 PM MDT this
     evening for UTZ478-492-497.




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