Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 171507
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
907 AM MDT Tue Apr 17 2018

.UPDATE...
Issued at 907 AM MDT Tue Apr 17 2018

Upgraded the High Wind Watch to a Warning over the far northeast
corner of Colorado. Latest, GFS, RAP, and HRRR all support gusts
to 60 mph after 00Z, behind the cold front. For the foothills and
South Park, wind gusts to 60-70 mph are expected, which is
slightly below the 75 mph High Wind threshold for these areas. If
it appears winds will be slightly higher will need to issue a
warning for these areas as well.

It`s been a warm start to the day with 9am temperatures in the
lower to mid 60s across the Denver area. Good chance temperatures
will be bumped up a little since the cooler air doesn`t arrive
until noon or slightly later.

&&

.SHORT TERM...(Today and tonight)
Issued at 421 AM MDT Tue Apr 17 2018

An upper level trough will move across the area mainly this aftn.
There is stg QG ascent and decent lapse rates associated with this
feature which will bring a quick burst of heavier snow to the mtns
this aftn with some thunder possible along with gusty winds.

Meanwhile, at lower elevations, sfc low pres will develop over nern
CO thru early aftn and then move eastward by late aftn.  Meanwhile a
pacific/bora type fnt will blast across the area nern CO by early to
mid aftn and exit the far nern corner by 00z. As this feature
moves across stg downward descent will also develop behind the
passage of the upper level trough.

Thus I would expect stronger flow aloft to mix down to the sfc.
700 mb winds are fcst to be around 50 kts or so will be close to
high wind criteria across most of the plains in the aftn. I would
expect some gusts up to 60 mph in places so will issue a warning.

Over the far nern corner confidence is not as high and time period
for high winds would be fm late aftn into early evening so will
leave a watch in place for now. Over the Palmer Divide high winds
could develop by sunrise so will issue a warning starting at 12z.
High wind threat over the Palmer Divide will likely end by late
aftn so will not have warning going past 6 pm. Finally am a little
concerned about South Park as some of the hi res data has gusts
exceeding 70 mph. Thus will have to keep an eye on that area later
this morning. In addition portions of the normal windy spots in
the Foothills (Boulder and nrn Jefferson) may have a brief 2 to 3
hour window of strong winds late this aftn, however, overall
coverage and duration could be limited so will not issue a
warning at this time but it will need to be watched as well.

As far as highs this aftn, temps early this morning were very mild
along most of the front range due to SWLY downslope flow.  With
expected downslope component, thru at least early aftn, highs may
rise more than previously thought so have raised readings over most
of nern CO.

For tonight snow will decrease in the mtns early this evening,
however, enough moisture will linger for sct -shsn thru midnight.
Across the plains the high wind threat should end by 02z in most
areas except possibly over the far nern corner.

.LONG TERM...(Wednesday through Monday)
Issued at 421 AM MDT Tue Apr 17 2018

Weakening zonal flow aloft is indicated on Wednesday as an upper
ridge moves across the CWA Wednesday night into Thursday morning.
For the rest of Thursday and Thursday night southwesterly flow
aloft moves in and increases in speed Thursday night. The QG Omega
fields show subsidence for the CWA on Wednesday, with benign
synoptic scale energy Wednesday night and the first half of
Thursday. By Thursday afternoon, upward energy is moving into the
CWA from the west. By 12Z Thursday morning, decent upward motion
is over the western forecast area. The boundary layer winds are
all northwesterly downsloping for the CWA on Wednesday. Late
Wednesday afternoon and evening, there are east and northeasterly
winds progged, then southeasterlies the rest of Wednesday night.
Increasing south-southeasterly low level winds are in place
Thursday and Thursday night. Looking at moisture, cross section
show it to be dry Wednesday and Wednesday night. Upper level
moisture increases on Thursday. By Thursday night moisture
increases from west to east, with it deepening over the mountains
by Friday morning. There is very little in the way of CAPE
through Thursday morning, with just a tad late day Thursday over
the foothills and immediate plains. The QPF fields have no
measurable precipitation on them Wednesday through Thursday
evening. There is some over the western and southern halves of the
CWA after 06Z Thursday night. For pops will bring them in
Thursday evening and night. For temperatures, Wednesday`s highs
are a bit cooler than today`s highs. Thursday`s highs warm up 3 to
7 C from Wednesday`s. For the later days, Friday through Monday,
the medium range models are showing an upper closed low over the
Four Corners on Friday afternoon. The feature is centered
somewhere from the Texas panhandle to southeastern New Mexico by
Saturday afternoon depending on the model. The persistence in the
medium range models for the last several runs showing this upper
low is uncanny. So it still looks like a decent precipitation
event for much of the CWA from Friday morning well into Saturday.
The latest 00Z model runs now show it warmer for night Friday and
Saturday than did previous model runs. The forecast builder
produces little to no snow now for the plains of northeast
Colorado Friday night into Saturday morning. We`ll see and
continue to monitor. There is weak upper ridging indicated for
Sunday and Monday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 907 AM MDT Tue Apr 17 2018

Main weather concern is winds through this evening. Southwest
winds gusting to 35 knots will become more WSW late this morning
with gusts up to 40 knots. After 18Z, winds turn westerly and are
expected to increase with gusts of 40 to 50 knots through 00Z. Due
to the strong winds there is a potential for reductions in
visibility due to blowing dust. Around sunset winds should
gradually decrease to 15 to 30 knots and continue to slowly
decrease overnight.

&&

.FIRE WEATHER...
Issued at 421 AM MDT Tue Apr 17 2018

Critical to extreme fire danger will exist across portions of the
area later this morning into early this evening.  After analyzing
the latest updates from a few of the fire managers have decided to
include the Denver metro area, South Park and the srn Foothills in a
Red Flag Warning just to be on the safe side.  Strong winds combined
with low humidity values will be in place later this morning thru
the afternoon hours so if a fire gets started it could spread very
rapidly.

There will be an elevated fire danger over Park, southern Lincoln
and Elbert Counties Thursday afternoon.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from 3 PM this afternoon to 9 PM MDT this
evening for COZ050-051.

Red Flag Warning until 8 PM MDT this evening for COZ239>241-
245>247.

High Wind Warning until 9 PM MDT this evening for COZ038-040-
042>049.

Red Flag Warning until 6 PM MDT this evening for COZ214-216.

High Wind Warning until 6 PM MDT this evening for COZ041.

&&

$$

UPDATE...Meier
SHORT TERM.....RPK
LONG TERM......RJK
AVIATION...Meier
FIRE WEATHER...RPK/RJK



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