Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 302117
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
317 PM MDT Thu Mar 30 2017

.SHORT TERM...(This evening through Friday)
Issued at 150 PM MDT Thu Mar 30 2017

Relatively quiet day today before attention turns to the spring
storm that will impact our area the latter half of Friday. A lee
trough across south central Colorado doesn`t move much throughout
tonight as pressure falls continue across western Colorado as the
potent storm continues to approach the Four Courners region. With
south-southwest flow aloft orographics are not great for most of
our mountains tonight but enough mesoscale lift exists for light
snow above 9000 feet. Do not expect more than an inch or two
overnight. Across the Plains, strong pressure falls across Western
Colorado generate a pressure gradient oriented along the Front
Range. The result will be strong east flow across the Plains and
Foothills throughout the day Friday. The upslope flow should keep
skies mostly cloudy to cloudy, and there is enough cool air east
and northeast of the state that temperatures will be 15-20 degrees
cooler on Friday compared to Thursday. Snow levels will start off
around 8000 feet, then as orographic lift and weak low-level cold
advection increase during the afternoon, snow levels will drop to
6200 feet or so late Friday evening. Best chances for
precipitation will be north of I-70, and best chance for
accumulating snow Friday evening will be across the Foothills of
Jefferson, Boulder, and Larimer Counties. Precipitation rates
should really pick up overnight into Saturday, as discussed below.

.LONG TERM...(Friday night through Thursday)
Issued at 150 PM MDT Thu Mar 30 2017

...Stormy weather pattern remains in place through early next
week...

Latest satellite imagery shows the next storm dropping southeast
through Nevada this afternoon. It will continue this track through
tonight and Friday, eventually reaching northeast Arizona by
Friday afternoon. From there, the low will likely continue to
slide east/southeast through northern New Mexico through Saturday.
The GFS continues to dip southward and not surprised here. NAM
still remains a far northern outlier and is discounted. Overall
preference still stays with the GEM/ECMWF blend with this
forecast.

For Friday night and Saturday, snow will become heavier in the
mountains and foothills but intensity will depend on the strength
of the upslope flow. Airmass is plenty moist with rich theta-e
axis pushing up against the Front Range, but prefer the lighter
upslope component pushing back against the Front Range as shown by
the Canadian/ECMWF. As a result, don`t think the TROWAL structure
will remain in place very long, and not nearly as long as the
GFS. Thus, the QPF amounts on the GFS still appear overdone. With
regard to precip type, appears the low level cold pool originating
over the Upper Midwest will be enough in combination with the
cold pool aloft to switch rain over to snow over most of the
plains Friday night. Switch should first occur above 5500 feet
early in the evening, and then down to 4500-5000 feet shortly
after midnight. With regard to accumulations, it will remain
difficult to say the least given the differences in each
particular model run and only slight variations in storm track,
and also the impacts of warm ground. We did taper off snow
amounts somewhat in lower elevations to account for the expected
track and lighter upslope, and some melting. Confidence is still
low for expected accumulations in elevations below 6000 feet.
Moderate to high confidence for the heavier snow of 6-16" for the
Front Range Mountains, Foothills, and Palmer Divide.

After dry and warmer weather Sunday into early Monday, we should
see convective showers develop in advance of the next storm system
moving southeast across the Central Rockies. Despite the more
progressive flow, this system does have potential to dig southeast
and create a short-live high impact storm. If storm does dig
similar to the Canadian, then several inches of snow would even be
possible in the lower elevations. Still a tremendous amount of
variation here, but forecaster experience suggests a deeper
solution than the GFS here, so again preferred a blend of
Canadian/ECMWF.

Then a longer period of drier and warmer weather should develop
through the latter half of next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 150 PM MDT Thu Mar 30 2017

Winds have been bouncing around this afternoon under the influence
of a weak surface pressure trough extending from the surface low
across South Central Colorado northward along the I-25 corridor.
Expect genearlly easterly flow at the TAF sites to continue this
evening. As southeast flow increases across the Plains tonight
and Friday morning, a Denver Cyclone should set up shifting winds
to the northwest at the TAF sites after 10Z/11Z. This shift in
winds could pull IFR stratus and possibly fog into DEN, and IFR
stratus into BJC and APA. Still some uncertainty on the low
ceilings/fog as high res models indeed develop fog/stratus across
the eastern Plains, but none of the guidance has latched onto a
Denver Cyclone solution yet. Have kept the stratus/fog in DEN TAF
and low stratus in BJC and APA Friday morning. Rain showers are
possible this evening, and enough instability is in place for an
outside chance of thunder. However, chances too low overall that
used VCSH in the TAFs. Major storm coming to Colorado Friday and
initially it will start off as rain Friday afternoon, changing to
snow at the TAF sites overnight into Saturday. WRKPSA has the snow
potential at DEN handled well.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Friday evening through Saturday
afternoon for COZ041.

Winter Storm Watch from Friday afternoon through Saturday
morning for COZ033>036.

&&

$$

SHORT TERM...Schlatter
LONG TERM...Barjenbruch
AVIATION...Schlatter



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