Flash Flood Watch 5, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
708 PM MDT Tue Oct 2 2018

.SYNOPSIS...The very moisture air across the forecast area will
fuel showers and isolated thunderstorms across the area through
Thursday. Heavy rain will be possible with any of the stronger
showers and storms. An upper level trough moving east across the
northern Rockies will push a cold front across Utah Thursday
night through early Friday. Significantly drier air will follow
the front on Friday.

&&

.DISCUSSION...
Early update this evening as its about to get busy with a
convective line approaching the Wasatch Front from the West
Desert.

Our 00Z KSLC RAOB measured the highest Precipitable Water and
surface dewpoint for the month of October at KSLC.

No matter how you measure it, the moisture is quite remarkable for
this time of year. Dewpoints have risen into the 50s and lower 60s
across most of the region. Precipitable Water values across
valleys south of I-80 are 1.00"-1.50".

Thermodynamics continue to favor heavy rainfall, with FV3 BUFKIT
forecast soundings indicating an LCL at or below 700mb, and a warm
cloud layer of 5-9kft.

A cold front associated with a strong system over the northern
Rockies continues to advance into far western Utah, providing the
dynamical support through moisture convergence, to take advantage
of the storm environment. No reason to think the line will
dissipate before impacting the Wasatch Front and points east late
this evening into the early overnight hours. In addition to heavy
rainfall and a flash flood risk, this activity will bring copious
lightning and wind gusts up to 45 mph. Small hail cannot be ruled
out in the strongest cores given the amount of effective shear
being observed.

Late tonight, model consensus is for convection to spread
northward across southwest Utah, associated with falling heights
ahead of the broad and deep closed low off the California coast.
Again, this activity will also bring a heavy rain and flash flood
risk.

Main part of this update was to reorient PoPs based on radar
trends, and the latest Convective Allowing Model guidance.

Tomorrow, while drying does occur, this should allow for better
destabilization and another round of locally heavy rainfall and
continued flash flood risk. The Flash Flood Watch may need to be
extended by the next shift.

&&

.PREVIOUS DISCUSSION...
The remnant of TS Rosa over northeast Arizona late this afternoon
will move through the four corners this evening, then shear off
to the east-northeast through Colorado later tonight. A broad area
of stratiform rain with embedded convection extending across
southeast Utah through western Colorado will eventually push out
of the forecast area later this evening. Lingering showers late
tonight will have a limited areal coverage and will produce little
additional rainfall.

The somewhat more unstable air mass brought on by strong heating
over west-central through northern Utah earlier today will likely
see a general weakening early this evening. This decrease in
convection looks to be short-lived as the apparent vorticity max
over western Nevada moves east-northeast across northern Utah
tonight. A band of low-level confluence underneath this passing
vorticity max with the entrance region of the jet just to the
north of the boundary will create a good synoptic situation for
precip to concentrate along and north of the confluence zone late
this evening through early Wednesday. Light to moderate precip
near this boundary could be an issue later tonight as this precip
sags south toward the burn scars in southern Utah county.

The upper trough currently positioned along the California coast
will turn east and move inland through the central/southern
California coast Wednesday evening, then weaken as it moves east-
northeast into and then through the Great Basin late Wednesday
night/Thursday. The areal extent and intensity of precip should
increase as the trough advances into the Great Basin late
Wednesday night. Widespread, and at times, moderate/heavy precip
will develop due to good synoptic-scale forcing and a still moist
air mass over the area.

A new wrinkle to forecast in form of snow will come into play
Thursday night/early Friday. A surface cold front with trailing
near 700mb temps dropping below freezing across northwest Utah
will arrive with the upper trough. This setup will drive snow
levels down enough to bring minor accumulations of snow to the
higher portions of the Wasatch and western Uintas.

Significantly drier air will trail the cold front on Friday. This
drier air should bring an end to this round of precip by late
Friday.

Active pattern continues through the entirety of the long term
forecast period. This should keep cool temperatures and
precipitation across the area through next weekend and into early
next week. A long wave trough will set up directly overhead and
slowly close off and then eject later Monday into Tuesday. The
next trough will follow a somewhat similar morphology, though
potentially further north and west. Given the extended time period
and relative lack of model skill in forecasting shortwaves
rotating around these features...kept long term pop grids roughly
the same.

Main message continues: below normal temperature and above normal
precipitation to continue into early next week.

&&

.AVIATION...
A line of thunderstorms to the west of the KSLC terminal will
arrive between 02-03Z. There is a 40% chance of outflow winds
greater than 26 kts between 02-05Z. A prevailing northerly wind
shift is also expected with the arrival of the storms. The
strongest storms will move east of the terminal by 05Z leaving
rain showers with embedded lightning until 09-10Z. VFR conditions
will prevail except in heavier precipitation when MVFR
visibilities are possible.

&&

.FIRE WEATHER...
The remnant of the tropical storm that spread an extensive field
of moisture across Utah will move northeast through the four
corners region this evening. Widespread moderate to heavy rain
will accompany this feature through southern and eastern Utah
tonight. Showers and isolated thunderstorms will exist across the
remaining fire districts overnight.

The upper level trough along the California coast will turn inland
late Wednesday, then lift northeast through the Great Basin and
weaken Wednesday night/early Thursday. A trailing upper level trough
over the northern Rockies will push a cold front across the state
Thursday night, bringing one more round of showers/storms
followed by significantly drier air to end the week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flash Flood Watch until 4 AM MDT Wednesday for UTZ001>004-
     006>016-019>021-517-518.

WY...Flash Flood Watch until 4 AM MDT Wednesday for WYZ021.

&&

PUBLIC...10/Conger/Kruse
AVIATION...Bonnette
FIRE WEATHER...Conger

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

Hazardous Weather Outlook
National Weather Service Salt Lake City UT
621 AM MDT Tue Oct 2 2018

UTZ001>016-019>021-517-518-WYZ021-031300-
Cache Valley/Utah Portion-Northern Wasatch Front-
Salt Lake and Tooele Valleys-Southern Wasatch Front-
Great Salt Lake Desert and Mountains-Wasatch Mountain Valleys-
Wasatch Mountains I-80 North-Wasatch Mountains South of I-80-
Western Uinta Mountains-Wasatch Plateau/Book Cliffs-
Western Uinta Basin-Castle Country-San Rafael Swell-
Sanpete/Sevier Valleys-West Central Utah-Southwest Utah-
Utahs Dixie and Zion National Park-South Central Utah-
Glen Canyon Recreation Area/Lake Powell-Central Mountains-
Southern Mountains-Southwest Wyoming-
621 AM MDT Tue Oct 2 2018

This Hazardous Weather Outlook is for the western two thirds of
Utah and southwest Wyoming.

.DAY ONE...Today and Tonight

Tropical moisture will continue to spread northward into the area
through tonight. As a result, much of Utah and southwest Wyoming
will see widespread rain. Some areas will see significant
rainfall amounts, with a potential for flash flooding
particularly in prone areas such as slot canyons, burn scars,
normally dry washes, small streams in steep terrain, and urban
areas with poor drainage.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Scattered showers and thunderstorms are expected Wednesday
afternoon, then rain will become more widespread Wednesday night.
Locally heavy rain will remain possible, and some thunderstorms
could produce flash flooding.

Another storm system will push a cold front into the area late
Thursday, bringing colder temperatures to the area along with the
potential for accumulating snow across the northern mountains.

Cool and wet weather will then continue for the weekend.

.SPOTTER INFORMATION STATEMENT...

Weather spotters are encouraged to report significant weather
conditions according to standard operating procedures.

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

For information on potential road travel impacts visit...
http://www.udottraffic.utah.gov/roadweatherforecast.aspx

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