Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 222102
Area Forecast Discussion
National Weather Service Salt Lake City UT
302 PM MDT Sat Oct 22 2016
.SYNOPSIS...An unseasonably warm southwest flow will exist across
the area this weekend. Moisture will spread north into Utah late
Sunday, and will continue to increase into early next week. Drier
conditions will return for the middle of the week.
.SHORT TERM (Through 12z Tuesday)...
Water vapor loop shows southwest flow aloft between a cold closed
low over the Gulf of Alaska and a strong ridge over Texas. Several
disturbances are shown embedded within the southwest flow aloft to
our northwest, with the strongest off the Pacific Northwest coast.
Another disturbance was over southeast Pacific Ocean near 25N
125W. MDCARS wind observations show a 125-140kt anticyclonic jet
from the Pacific Northwest into south central Canada and down the
western Great Lakes. GOES/HRRR/12Z KSLC RAOB indicate the
precipitable water value ranges from 0.15"-0.25" mountains to
0.30"-0.40" valleys. Blended Precipitable Water product shows a
moisture plume beginning to surge north from the Gulf of
California. A second plume of moisture was off the Pacific
Northwest coast extending back into the central Pacific where it
is substantially greater in magnitude.
Active weather pattern in the short term.
Breezy southwest flow with good mixing south of Interstate 80,
where a shallow cold front has settled. This boundary will lift
northward later tonight in response to increasing flow as the
This also results in drawing up a moisture surge from the Gulf of
California. With afternoon heating, expecting the potential for
isolated showers primarily across the west and south. Rainfall
will be quite limited with showers being high based. Locally
enhanced winds are possible as the result of dry microbursts.
Outside of showers, expecting another breezy day for much of the
area with thickening cloud cover moving into southwest and west
central Utah. This should clear out the urban haze as tomorrow
The moisture surge arrives even across the north by tomorrow
evening. Increasing diffluence aloft will combine with increasing
instability seen on the SREF to promote an increase in coverage of
showers. Went above typical blend of guidance for overnight lows
across northwest Utah given gusty southerly flow and cloud cover.
Subtropical jet from southern Pacific interacts with the jet
downstream of the trough to our northwest Monday, with coupling
evident between the left exit region of the subtropical jet and
right entrance region of the northern jet. This will be favorable
for showers and thunderstorms becoming widespread, along with
organization considering the good chance of at least 30kts of
effective shear. Given the moist profiles, with warm cloud layer
of around 5kft, heavy rainfall is a concern. There is still some
uncertainty as to the corridor for the bulk of this threat, but as
of the 12z guidance, it looks to extend from near Cedar City to
Evanston plus about 50 miles to the northwest and southeast of
this line. This will become better refined by tomorrow. As for
temperatures Monday, cut temperatures a good amount from typical
model blend in the vicinity where the most convection is expected
as outlined above.
A strong Pacific jet arrives into California late Monday, nosing
into western Utah by 12z Tuesday. The associated upper level
trough crosses Utah Monday night. Expecting this feature to shunt
the active weather to the east as the night progresses. This
results in some drying as heights rise to the west.
Snow Levels look to be around 10ft through the short term period,
potentially a bit lower during best precipitation rates.
.LONG TERM (After 12z Tuesday)...
The mid and upper levels will trend to dry from the
west/southwest Tuesday as in increasingly strong near zonal jet
penetrates the western Great Basin, this driving the primary
moisture tap from the south further downstream. BUFRs continue to
depict a slower drying trend at the low levels however, and with
increasingly upper diffluent flow aloft aiding lift across the
north, have maintained scattered shower potential across the
north/east through the day before drying further Tuesday night.
The pattern thereafter amplifies overhead and upstream as the
predominant eastern pacific trough winds up off the coast and mid
level ridging builds over the intermountain region. Globals in good
agreement through Wednesday depicting the ridge axis over eastern
Utah north into Alberta, prior to shifting downstream Thursday
allowing an increasing southwesterly flow aloft to transition over
the eastern Great Basin. Have maintained a dry forecast through this
period with temp trends climbing through Thu (supported by both WAA
aloft and decent mixing potential).
Globals thereafter continue to depict the core of the upstream
trough getting kicked NE by another upstream, then becoming
negatively tilted over Nevada prior to lifting through the forecast
area sometime Friday. Models continue to support a strong moisture
tap with this system, though the primary upper level feature will be
weakening overhead. Timing issues continue regarding passage with
the EC slightly faster than the slower GFS, but enough consistency
exists for widespread PoPs Friday. Its more a question whether the
bulk of the precip will occur in the morning or the afternoon.
Regardless the western half of Utah should be favored.
Heading into next weekend have minimal confidence of the next
trough`s evolution and the long wave pattern as a whole. We should
see a net drying trend on day 7 between troughs, but expect run to
run consistency in models to remain poor in this dynamic pattern.
Northerly will continue at the KSLC terminal through the rest of
the afternoon then switch to southerly around 02-03z. The
southerly winds could become gusty after about 06-08z but gusts
could hold off until 14-16z. VFR conditions will prevail under
increasing high clouds.
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