Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 261003
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
403 AM MDT Wed Apr 26 2017

.SHORT TERM...(Today and tonight)
Issued at 402 AM MDT Wed Apr 26 2017

Satellite shows upper level trough that produced yesterday`s
convection moving off quickly to the east. There was some stratus
backed up against the Front Range but satellite is now showing
some erosion as subsidence and more of a downslope northwesterly
flow kicks in.

Flow aloft will transition today and tonight to stronger
northwesterlies. Another short wave embedded in this flow will
drop southeast from the Pacific Northwest this morning into
northern Colorado late this evening. This is shown nicely in the
Q-G fields with weak to moderate lift overnight into Thursday
morning. Mid level moisture appears to be increasing ahead of
this feature with clouds already forming on the western slope.
Moisture will continue to increase through the day, with snow
showers becoming more widespread in the mountains by this
afternoon. Some of these may be heavy with embedded convection and
isolated thunderstorms. Steadier snowfall is expected to develop
overnight with the best lift and strong cold advection reaching
the mountains late tonight into early Thursday. Expect most
mountain areas to pick up a burst of heavier snow during this time
frame, and widespread enough accumulations and sufficiently cold
temperatures to produce travel impacts including I-70 through the
mountains tonight through Thursday. Therefore issued a Winter
Weather Advisory for this period, with accumulations expected to
reach the 4-8 inch range with locally heavier amounts upwards of 1
foot due to convection.

On the plains, should see some sunshine return today before
convective cloudiness increases this afternoon. Various models
suggest a weak convergence zone over the eastern plains with a
little better chance of afternoon showers/isolated storm there.
Some showers may also drift off the higher terrain despite the
slightly drier airmass along the I-25 corridor. Chance will remain
in place on the plains, and may actually increase later tonight
with best Q-G lift arriving.

After a below freezing start to the day, temperatures will be able
to warm into the mid to upper 50s across the plains. That`s still
several degrees below normal.

.LONG TERM...(Thursday through Tuesday)
Issued at 402 AM MDT Wed Apr 26 2017

...Unsettled weather pattern through early next week with
accumulating snow down onto the plains possible Friday night into
Saturday...

Showers should become more widespread at least on the northeast
corner of Colorado Thursday morning with the best lift and mid
level moisture convergence there. Snow will continue in the
mountains during the morning, but gradually taper to showers for
the afternoon. Airmass is still slightly unstable so isolated
thunderstorms again expected in the afternoon. Temperatures will
remain below normal with highs similar to those observed today on
the plains with 50s, but only upper 20s to 30s mountains with
colder air aloft arriving.

On Thursday night, we should see some drying overall with passage
of the lead short wave, although scattered light snow showers
will continue across the mountains.

Attention will then turn toward the next Pacific jet streak,
which will aid digging of the upper low toward the Four Corners
region late Friday into Saturday. Models have been rather
consistent with the storm location and evolution, but the details
are yet to be worked out. It still appears models are all cold
enough to produce almost all snow even into the lower elevations
of the plains by late Friday night and Saturday. ECMWF continues
to pump out the most precipitation from the operational runs,
although the GFS has trended toward that solution as well over the
last 24 hours with more of an easterly mid level component for
the Front Range. The latest runs and ensemble averages still
suggest accumulating snowfall across the area, and see no reason
to argue with that considering the overall pattern appears to be
well modeled. However, the details is where this storm will get
difficult. If light snow falls, then relatively warm ground and
sun angle could limit accumulations. If heavier snow falls in a
stronger upslope regime with more significant accumulations, then
we could be looking at a high impact event given many trees are
leafing out already. Still considerable uncertainty as to which
scenario wins out, so will continue to refine the forecast over
the next couple days. Spring storms are almost always difficult
and this one should be no different. Will word the weather change
accordingly in the Hazardous Weather Outlook this morning.

Then look for drying and warming Sunday and Monday, although there
is potential for a weak short wave to bring light snow to the
mountains. Confidence is increasing for more rain/snow as we head
into Tuesday with another storm system dropping in from the
northwest. Again, there appears to be enough cold air with this
one to bring potential snow accumulations down onto the plains.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 402 AM MDT Wed Apr 26 2017

Areas of stratus with ceilings 1500-2000 feet across the Denver
metro area airports are finally beginning to break up early this
morning. Flow in the stratus deck is finally shifting more
north/northwesterly so that will help it break through 12Z.
After the last of the stratus breaks, VFR conditions will then
prevail through tonight. Low chance of showers (20-30%) through
tonight, with just a 10% chance of a passing thunderstorm.
Ceilings gradually lowering to around 8000 feet with increasing
moisture 00Z-12Z.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Thursday for COZ031-033-034.

&&

$$

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



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