Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 221612
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1012 AM MDT Thu Jun 22 2017

.UPDATE...
Issued at 1010 AM MDT Thu Jun 22 2017

Northerly surge showing up nicely on water vapor satellite
images, as well as in surface observations. Main pressure rise is
pushing southeast into central Nebraska, with temperatures in the
upper 60s to lower 70s there and back into Wyoming. Readings are
cooling a few degrees in Colorado as the first push arrives, with
northerly winds gusting up to 30 mph and dewpoints in the upper
40s and 50s.

Today`s forecast still on track for expecting the increase in
moisture combined with upslope flow this afternoon to contribute
to the potential for severe weather - mainly large hail and strong
outflow winds. The better upslope and moisture is still expected
over the northern half of the forecast area, whereas drier air to
the south will likely lend itself a little more toward the strong
outflow winds. Latest HRRR and experimental HRRR runs showing
cells producing outflow winds in excess of 50 kts, mainly south of
I-70, where the drier subcloud air will be. However, confidence
in placement and timing in these models are not very high yet
today, as earlier runs have been very inconsistent with the front
as well as potential convection this morning. Yet the potential is
still there to warrant the SPCs Slight Chance category into the
Front Range.

&&

.SHORT TERM...(Today and tonight)
Issued at 329 AM MDT Thu Jun 22 2017

Lots of things going on today. We will start warm and dry again,
and temperatures will likely be similar to yesterday morning`s
warmup. A pressure surge dropping across northern Wyoming is well
ahead of the real temperature gradient that is still back in
Montana. This will continue to run southward and will bring the
air now in Wyoming into Colorado this afternoon. While there will
be slight cooling, this air has more moisture and will continue to
moisten into this evening, entraining richer moisture from areas
further east by evening. At first this moisture will be capped as
the temperatures aloft will change very little. However,
additional moistening and the peak heating will weaken the cap.
Upslope winds on the foothills should initiate convection in the
mid afternoon hours, and this should then provide enough
convergence for storms to propagate eastward through the late
afternoon and evening. It is looking like the best
moisture/instability will be over our area in the late afternoon
and evening. Temps in the mid 80s with dew points in the upper 40s
will produce CAPEs of 1000-1500 J/kg, and 40-50 knots of surface-
6km shear should provide a favorable environment for large hail
production. With the expected wind profile, we will likely begin
with left-moving supercells as the preferred storm mode,
transitioning to splitting storms or more likely outflow-driven
linear storms after a few hours. This means the greatest severe
threat will likely be in the first few hours of convection and
therefore nearer to the Front Range, even extending into the early
storms on the foothills. Not sure if there is enough CAPE for very
large hail, but given the shear we may be able to make the best of
the environment. While hail may be the main threat, plains storms
will likely get wet enough and organized enough for a wind threat
as well.

Moisture will continue to feed in from the east this evening,
likely regenerating showers and storms after the initial batch.
The strong temperature gradient should arrive as another surge of
northerly winds during the night, bringing significant cooling.
There will probably be some showers with the cooling and
convergence with this as well during the night, but this area
should be moving south of us by morning. Temperatures in Montana
are in the 40s and models are showing stronger cooling than what
we were forecasting, so I lowered tonight`s lows to around 50 on
the plains.

.LONG TERM...(Friday through Wednesday)
Issued at 329 AM MDT Thu Jun 22 2017

On Friday, the flow will be northwesterly with the cwa under the
influence of systems passing to to the north and east. The next
more significant cold front will push across the northeast plains
early Friday morning. The airmass following fropa is progged to
become increasingly stable through the day. Not much in the way
of boundary layer CAPES to work with as the post frontal airmass
quickly stabilizes. The best chc of any showers or thunderstorms
Friday afternoon and evening will be over the elevated terrain
primarily to the west and southwest of Denver, in and around South
Park. The models also show quite of bit of stratus along the
Front Range and adjacent plains Friday morning. On Saturday,
another surge of cool air will push across the northeast plains
further capping an already stable situation. As a result, focus
for most of the showers and thunderstorms will continue to be over
the elevated terrain. It will be a little warmer on Sunday but
still 5-8 degrees below normal. The GFS buffer soundings indicate
enough warming Sunday afternoon to possibly break the cap, with
values in the 700-900 j/kg range. Confidence in this is low at
this time so will keep the the mention of showers and
thunderstorms to areas in and near the Front Range Foothills and
Palmer Divide. For next week, the upper level ridge is progged to
build over the western U.S. so will continue with the dry and
warmer trend, with temperatures climbing back to around 90 by
Tuesday. Increasing mid level subsidence associated with the ridge
with limited thunderstorm potential, isolated at best and
primarily over the mountains.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1010 AM MDT Thu Jun 22 2017

Front will move through the area airports within the next couple
hours, with winds becoming northeast with gusts in the 20-30 knot
range around 16-17z. Scattered strong thunderstorms are expected
to develop this afternoon, with the main threat between 21z and
03z. Localized MVFR ceilings and IFR visibilities along with hail
are possible with the strongest storms. Multiple wind shifts with
gusts as high as 40 knots are also likely in this period. For
tonight, ceilings will lower, with areas of MVFR-IFR conditions
developing as moist upslope flow continues through the night.
Ceilings of below 1000 ft AGL may remain through 18z Friday.


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

UPDATE...Kriederman
SHORT TERM...Gimmestad
LONG TERM...Cooper
AVIATION...Gimmestad



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