Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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481
FXUS65 KBOU 141025
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
425 AM MDT Wed Apr 14 2021

.SHORT TERM...(Today and tonight)
Issued at 425 AM MDT Wed Apr 14 2021

Water vapor shows a broad trough over the Great Basin this
morning with a few small shortwaves rotating around it. One of
these shortwaves is moving over northern Colorado currently and
the associated warm air advection with it is producing snow
showers across the far northern mountains as well as portions of
Larimer and Weld Counties. These snow showers may linger until
shortly after sunrise but they will move out of Colorado by the
mid morning.

Another thing water vapor reveals is the substantial dry air that is
over much of the Desert Southwest. It is this dry air that will move
over our forecast area during the day today that lead to the
majority of locations staying dry for the entire day. PoPs were cut
drastically because there will not be enough lift to overcome the
dry air. From I-25 to the eastern plains on Colorado, I took out the
mention of rain/snow showers for today. Considering the lack of
precipitation and the likelihood that clouds will dissipate over
much of the area, high temperatures were raised about 5 degrees
above the previous forecast. A high in the low 50s is well within
reason in Denver.

The tricky part of the forecast will be the possibility for
precipitation to develop later this afternoon and evening across the
mountains and foothills. With the elevated mixed layer moving
overhead along with relatively strong heating, decent instability
will develop. Low to mid level lapse rates will be around 9 C/km
with mixed layer CAPEs in the 500 J/kg range. So if convective
showers were to develop, they would be intense with brief heavy
snowfall/graupel. The issue will be whether convective initiation
can even occur due to the weak forcing. Recent high resolution
model runs have been backing off on convection in the mountains
indicating the forcing will not be strong enough to overcome the
dry air. With that being said, I wonder how well the models are
picking up on the dry intrusion that is currently moving through
southern Utah and northern Arizona. If this feature were able to
move across the central mountains of Colorado around peak heating,
it could provide that extra lift for convective initiation to
occur. Healthy PoPs were kept in the mountains and foothills to
indicate the possibility for good coverage of showers late this
afternoon and through the overnight hours. Showers will eventually
make their way onto the urban corridor and plains this evening
and overnight tonight but since there will be less instability
farther east, any precipitation that falls should be on the
lighter side. 1 to 5 inches of snow are possible in the mountains
and foothills with minimal snow accumulation across lower
elevations.

.LONG TERM...(Thursday through Tuesday)
Issued at 425 AM MDT Wed Apr 14 2021

With the trough still upstream of Colorado, Thursday will
continue to be cool with increasing chances of precipitation.
Models are in good agreement that the trough will slowly move east
across the northern half of Colorado Thursday into early Friday.
QG lift ahead of the trough is maximized Thursday afternoon first
across the east slopes and I-25 corridor then spreading east
across the northeast plains. With a very strong downstream trough
across New England on Friday and a massive ridge across the PacNW,
the movement of the trough over us will be very slow. In fact, a
piece of the trough looks to hang back just to our west most of
Friday into the first half of Saturday with a positive tilted
trough axis going southwest to northeast across Colorado. This
means cool and wet conditions are likely to continue until early
Sunday. Details are always important, especially in terms of what
to expect in terms of impacts. The best QG lift is from Thursday
afternoon through Friday midday. At the mesoscale, strong
frontogenesis occurs east of the Divide late evening into early
Friday. These two factors alone should lead to moderate
precipitation rates resulting in QPF of a few tenths to over a
half inch of QPF Thursday afternoon through Friday midday for much
of the area east of the Divide. Surface-700 mb wind direction is
north for the most part, but starts off northeast for several
hours on Thursday afternoon and evening. The primary surface low
develops across E. NM late Thursday and moves east, weakening
along the way as the parent trough ejects much further north.
This is the main reason for more northerly winds across the I-25
corridor. The surface to 700 mb low position should drive a cold
front across our area thursday afternoon, with temps plummeting to
-11 degC at 700mb and the upper 20s at the surface across the
 plains by Friday morning. PW values should be in the 0.4-0.6"
 range, not too shabby for mid-April and with a cold storm. All
 told the best snow rates will begin across the west slope
 Thursday morning into afternoon, spreading east afternoon and
 evening, and continuing across the eastern plains into Friday
 morning. Rates should remain below 1" an hour with lapse rates
 weakening by Friday morning. In terms of P-type, the west slope
 valley and east slope foothills and urban areas nearest the
 foothills should be all snow. Some rain should occur east of I-25
 during the afternoon, but even there it should change over to
 snow by sunset Thursday. From Thursday afternoon through Friday
 morning advisory level snow amounts are possible especially east
 of the Divide, but travel impacts should remain minor with modest
 snow rates expected. With a piece of the trough still upstream on
 Friday, expect very light precipitation (most likely snow) across
 the area but very little additional accumulation. Any travel
 impacts should be just about gone mid-morning with the April sun
 angle despite temps in the 30s across the plains and mostly
 cloudy skies. Highs should reach close to 40. In the mountain
 valleys, expect highs in the low to mid 30s. Despite a lull in
 the precipitation east of the Divide on Friday and Saturday, the
 central mountains along and south of I-70 may pick up a few
 inches of snow via QG lift and some frontogenesis along the
 remnant trough axis. There is no upslope and lapse rates are
 weak, but there are enough ingredients in place for continued
 light snow through Saturday. Additional snow across the northern
 mountains Friday and Saturday shouldn`t be much, and the plains
 could just see flurries. Temps don`t rebound much on Saturday
 with decent cloud cover and continued cold advection aloft. For
 now it`s looking like low 40s for the plains and 30s in the
 mountain valleys and eastern foothills.

If you couldn`t tell by the discussion above, overall confidence in
the forecast is pretty low.  Confidence in Sunday`s weather is much
higher, given the remnant piece of the trough looks to slide south
(too far) of our area, leaving us in northwest flow aloft and some
warm advection. PW drops to 0.2" and with dewpoints in the teens, no
precip is expected.  Temps should rebound into the 50s across the
plains, with 40s in the high country.  There is one pretty important
potential impact to pass along, and that`s the low temperature
Sunday morning around sunrise.  Only the GFS/GEFS keeps clouds
around and some lingering low-level moisture Sunday morning, and
thus low temps are not overly cold.  The Canadian and EC ensembles
really dry it out at the low levels and clear us out. Though not
much snow cover is expected across the plains and I-25 corridor,
any snow cover combined with ideal radiational cooling would
result in very cold lows indicated by a about half of the
ensemble member. Lows around 10-15 degF are not out of the
question along the I-25 corridor Sunday, and thus we have begun
the process to undercut blended guidance to account for this
potential trend for Sunday morning.

Monday and Tuesday looks cool and unsettled again as ensemble
solutions bring a trough in the northwest flow across the northeast
quadrant of Colorado.  Depending on timing, some rain could occur
Monday afternoon below 7,000 ft, but all model guidance for now
shows a strong cold front sometime in the latter half of Monday
which would result in snow as the p-type into Tuesday morning.  It
looks to be a quick moving system with moderating temps and drying
during the day on Tuesday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 425 AM MDT Wed Apr 14 2021

A low to mid level cloud deck will be the only concern for much of
the day today. ILS ceilings are likely through most of the TAF
period although ceilings could break up around midday today. Winds
will generally be light but a southeasterly surge in the afternoon
may produce wind gusts up to 25 knots. It seems with every model
run the chance for precipitation decreases for the terminals.
Only light precipitation in expected with minimal visibility
reductions tonight. Ceilings may lower into the MVFR range
tonight.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM MDT early this morning for
COZ033>035.

&&

$$

SHORT TERM...Danielson
LONG TERM...Schlatter
AVIATION...Danielson



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