Area Forecast Discussion
Issued by NWS Salt Lake City, UT

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
000
FXUS65 KSLC 301600
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
1000 AM MDT Fri Sep 30 2016

.SYNOPSIS...Abundant moisture over Utah will fuel additional
showers and isolated thunderstorms again today. A cold Pacific
storm system will enter the Pacific Northwest this weekend, then
move southeast in Utah during the early portion of next week.

&&

.DISCUSSION...
Water Vapor loop shows waves shearing into the region over
northwest Utah and southeast Utah, ahead of a broad cold closed
low off the Pacific Northwest coast. MDCARS winds observations
show a 60KT-90KT southwesterly jet over the four corners, and a
second cyclonic jet of similar magnitude over the Pacific
Northwest and western Canada. GOES/HRRR/12Z KSLC ROAB indicate
precipitable water values range from 0.30"-0.50" mountains to
0.80"-1.15" most valleys.

Primary concern today is once again the threat of locally heavy
rainfall across the eastern valleys. Moisture is still abundant as
accessed by precipitable water values and surface dewpoints in
the 50sF. Slow storm motions could enhance the threat even
further, though this also means storms may be closer tied to the
higher terrain in less flashy areas. We will obviously have to
monitor these conditions closely. For now added mention of heavy
rainfall to the grids for these areas.

Otherwise added isolated PoPs across northern Utah this morning to
account for weak elevated showers associated with the further
northwest jet feature.

Otherwise adjusted temperatures, RH, winds, and QPF based on
latest observational data and model guidance, though changes for
the most part were minor.

&&

.PREVIOUS DISCUSSION...
Isolated showers early this morning will become better organized
and expand in areal coverage as two weak dynamic feature impact
Utah today. The first and more obvious feature is the southern end
of the shortwave ejecting northeast out of the cold core upper
trough off the Pacific Northwest coast. Radar and satellite
imagery indicates an area of loosely organized convection across
northeast Nevada. This convection will work through northwest Utah
this morning. Considering the weak dynamic and thermal support for
lift, see little chance for any significant precip to develop in
association with this feature today.

A second more subtle dynamic feature moving slowly east along the
Utah/Arizona border may be a bit more promising as it will have
substantial deep layer moisture (PWAT values of 1.00 to 1.25
inches) to work with today. Suspect that areas of south-central
through east-central Utah will see the potential for locally
heavy rain again today, especially with any convection that might
get hung up on the terrain late this morning through mid-afternoon.
Convection will likely trend downward late in the afternoon as
the shortwave exits east into Colorado.

The cold core Pacific trough will move into the Pacific Northwest
this weekend, then track southeast into the western Great Basin
Sunday night. Will keep an eye on northwest Utah in terms of an
increasing threat for convection Saturday night/Sunday. The
associated baroclinic zone aloft will shift east ahead of the
advancing upper trough, with this baroclinic zone settling into
northwest Utah Saturday night. This boundary will likely be the
focus of convection as shortwaves eject northeast out ahead of the
main trough. Have increased Pops across much of northern Utah in
the morning forecast package.

Gusty south winds look to be a good bet this weekend as the near
700mb winds increase ahead of the advancing upper trough. Will
likely see winds strong enough for wind highlights across southern
and western Utah by Sunday.

Big picture consistency in global models regarding the early week
upper low continues, though subtle differences in amplification
and evolution of the cold pool depth and track do exist.
Regardless, this trough is quite cold and dynamic, and will bring
another marked shift to the weather Monday and
Tuesday...especially across the north.

A fairly strong cold front will work west to east across the area
during the day Monday, this as the cold core digs into and through
Nevada. A strengthening upper jet rounding the base and setting up
overhead on the downstream side coincident with frontal passage will
aid strong deep layer lift, but breadth of deep layer moisture will
be much less than last week`s upper low owing to less prolific
precipitation than that which was recieved. Do expect widespread
precip and have retained likely PoPs for the period from frontal
passage (Mon) through axis passage (and a period of upglide Tue)
across the north, but run total QPF looks less than half in
comparison to the previous low. Southern Utah and the east, much
less.

Aforementioned differences in cold core depth and track do have a
net impact on confidence regarding the aforementioned upglide period
Tuesday, as the EC suggest a last weak wave within NW flow limiting
the potential, though maintains a colder more unstable airmass as a
whole into Tuesday Night while heights build more rapidly in the
GFS. Will let these details fall out in subsequent runs, and have
generally maintained a fairly rapid drying trend heading into
Wednesday.

No change in the outlook for high elevation snow Mon/Tue across
namely the northern mtns. H7 temps pushing -6C are expected to
support some snow down to or around 6500ft, with modest
accumulations focusing over 8000ft.

A relatively tranquil northwesterly flow aloft will dominate the
local weather mid/late week, with only a slow rebound in temps
through the end of the long term. Temps some 15 to 20 degrees below
normal early week should only rebound to about 5 to 10 degrees below
normal by late week.

&&

.AVIATION...
The SLC terminal will see VFR conditions through the afternoon
with any cigs remaining above 6000 feet AGL. Southeast winds are
expected to shift to the northwest between 19Z and 21Z. There is a
30 percent chance this shift will occur later than expected.

&&

.FIRE WEATHER...
Yesterday`s ERC values were below the 50th percentile across the
state.

Convection will affect mainly areas along and east of the spine of
the mountains today. With moist profiles, the threat for heavy
rainfall will exist through this evening.

Significant storm system develops along the west coast this
weekend, expecting to see an increase in southwest winds and
lowering RH values. Critical fire weather conditions due to gusty
winds and low RH cannot be ruled out Sunday where fuels are deemed
critical.

The storm system crosses Utah early next week, bringing a cooling
and moistening trend with a good shot of wetting rain/high
elevation snow across northern and portions of central Utah.

&&

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

$$

PUBLIC...Rogowski/Conger/Merrill
AVIATION...Traphagan
FIRE WEATHER...Rogowski

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



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.