Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 222111

Area Forecast Discussion
National Weather Service Denver/Boulder CO
311 PM MDT Wed Mar 22 2017

A Significant Precipitation Event Looks Likely Thursday Night
Through Friday...

.SHORT TERM...(This evening through Thursday)
Issued at 232 PM MDT Wed Mar 22 2017

Southwest flow aloft will continue through Thursday as a potent
Pacific storm moves onshore across Southern California. The storm
is currently located near Santa Barbara and will move east across
the lower Colorado River Valley tonight. The strongest part of
the jet stream will be on the east side of the trough, meaning it
shouldn`t dig any further south on Thursday. Models have it moving
east-northeast into the Four Corners by late Thursday. The storm
will advect moisture along southwest flow throughout the day, but
with an exceptionally dry atmosphere at the surface and initially
aloft, chances of precipitation should hold off until mid-morning
Thursday in the mountains, and until Thursday afternoon across
the Plains. Temperatures start out warm all areas and any
precipitation that falls Thursday should be rain below 9500 feet
MSL. QPF should be light through Thursday afternoon and the
upslope and synoptic scale lift hold off until later. Very
limited instability is available so have not made a mention of
thunder, though any precipitation that develops will be showery
(brief, widely scattered) in nature.

Biggest concern before the significant precipitation kicks in
Thursday night will be fire weather. Strong southwest flow aloft
will result in lee troughing that kicks east throughout the day.
Strong mixing via daytime heating will transport the southwest
winds down to the surface, especially the southern half of the CWA.
Please see the fire weather discussion below for details. The
southwest downslope flow will result in high temperatures in the
low 70s despite ample cloud cover throughout the day.

.LONG TERM...(Thursday night through Wednesday)
Issued at 232 PM MDT Wed Mar 22 2017

Cold front will race down the plains Thursday evening with snow
continuing over the mountains and rain showers filling in behind
the front over the plains. Model differences on the upper low`s
position and resulting temperatures, wet bulb zeroes and wind
direction is continuing to lower confidence on storm impacts over
the elevations below 7000 feet. The NAM continues to be the
further south solution, though its last run trended north, and has
colder temperatures but also more downsloping over the lee of the
divide. With it being the outlier model, did not use much
influence in the forecast. Will trend it toward more of the GFS
and ECMWF at this point, which shows snow levels closer to 6000
feet by early Friday morning, and has more upslope direction
winds. With moderate to strong upward QG values expected to move
over the area late Thursday night and into Friday morning, along
with some good isentropic lift over the plains will result in good
precipitation over most of the area in a short 12-18 hour time
frame. QPF amounts of 0.5 to 1.2 inches are expected. Tough issue
is the rain/snow line. Looks to lower to around 5500-6000 ft by
6am Friday, but then looks to lower slightly more during the
morning to perhaps 5000 feet before rising back up that
afternoon...with downsloping and the system kicking out east. The
urban corridor, not including the Palmer Divide, will be tricky.
All in all, it will be a wet storm, and elevations around 6k-7kft
will see heavy wet snow. The warm temps will limit any
accumulation of what snow does fall. Also, the northerly winds
will likely keep areas of Boulder and northward warmer and
therefore more rain chances than snow. The mountains, southern
foothills and Palmer Divide above 7000 feet will have a Winter
Storm Watch issued for them for the 18 hours between Thursday
evening until Friday afternoon where 8 to 15 inches may occur.
Precipitation will quickly end from west to east Friday afternoon
through Friday evening.

Another impact will be the strong winds on the backside of the
system, spreading strong northerly winds over the plains Friday with
sustained 30 to 45 mph expected.

Drier and warmer weather is expected on Saturday. The next system
is progged to move across CO midday Sunday so another chance of
mountain snow with rain across northeast plains Sunday
afternoon/evening. Drier Sunday night into Monday, with another
chance of precipitation for the region Monday night through
Wednesday of next week.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 232 PM MDT Wed Mar 22 2017

VFR conditions through Thursday evening. Winds should come around
to the south-southwest at APA and DEN this evening after 02Z or
so. Winds will pick up out of the southwest after 14Z on Thursday
as a surface trough moves east into the far eastern Plains of
Colorado. Winds will be strong and gusty much of the day Thursday
at all three TAF sites, but should be strongest at DEN. A strong wind
shift to the north will occur a few hours after 00Z Friday,
lowing ceilings and increasing precipitation chances.


Issued at 232 PM MDT Wed Mar 22 2017

Fuels continue to be exceptionally dry as evidence by several
fires that broke out today across the foothills southwest of
Denver despite modest winds at the surface. Southwest flow aloft
on Thursday will mix down to the surface Thursday afternoon,
resulting in strong, gusty winds across the southern half of the
Plains and across the southern Foothills. Temperatures will be
much above normal with low 70s Plains and near 60 across the
foothills. A red flag warning is in effect for Thursday afternoon
for winds and low relative humidity across the southern half of
the county warning area and along and east of the foothills. Late
Thursday night winds will shift to the north and gust over 30
mph, but the good news for fire weather is that the front will
also usher in very moist low-level air and dramatically increase
the chance of precipitation area-wide Friday. A significant
precipitation event with snow above 6000 feet elevation is
expected much of Friday.


Red Flag Warning from noon to 6 PM MDT Thursday for COZ216-

Winter Storm Watch from Thursday evening through Friday morning
for COZ033-034-036-041.



SHORT TERM...Schlatter
LONG TERM...Kriederman
FIRE WEATHER...Schlatter is the U.S. government's official web portal to all federal, state and local government web resources and services.