National Weather Service Forecast Discussion

453
FXUS65 KBOU 231454
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
854 AM MDT Fri Aug 23 2019

.UPDATE...
Issued at 843 AM MDT Fri Aug 23 2019

Main concern again revolves around the severe weather
threat over the northeast plains this afternoon and early evening.

On the large scale, water vapor imagery showing upper trof centered
over northwest Wyoming with tailing trof over northern Utah. This
disturbance will approach Colorado this afternoon with weak to
moderate upward ascent noted in QG fields. As for moisture levels,
precipitable water values have dropped over the Front Range, with
values around a half inch and results in a drop of 1/4 inch of water
in past 24 hours. 12Z DNR sounding showed an increase in moisture
above 500mb but drier low levels. Surface dewpoints are only in the
40s over the urban corridor but still holding in the 50s and lower
60s across the far northeast plains. This moisture likely to hold
in modest southeast winds.

Like yesterday, Front Range impacts will be the threat of gusty
winds as forecasts showing inverted-v type soundings with the
drier low levels. As these outflow boundaries push east and into
the richer air, storms should intensify with ML CAPE values of
2000- 3000j/kg. So overall, severe weather threat looks to be
along a line from Limon to Greeley and points east. Last few runs
of the HRRR showing a broad linear feature of storms has they push
eastward through the evening hours. This pretty much the target
area that SPC has indicated for a slight chance of severe risk.
Any severe risk further west would be mainly confined to a strong wind
threat. Given the amount of mid/high level moisture around the
area this morning, this may hamper the heating a bit which may
cut down some on the instability. Overall a similar situation to
yesterday. Main threat will be hail, damaging winds but can`t
rule out weak landspout tornado given boundary interaction
expected.

&&

.SHORT TERM...(Today and tonight)
Issued at 439 AM MDT Fri Aug 23 2019

Southerly low level winds are covering much of the CWA at this
time. The convection had moved eastward out of the CWA shortly
after midnight. The mid and upper level cloudiness over the CWA
looks slow in dissipating at this time. Models are still showing
an upper trough to move across the CWA on Friday into the evening
hours. The flow aloft will be southwesterly Friday into the evening,
then northwesterly overnight Friday night behind the upper trough
axis. The QG Omega fields do show some upward synoptic scale
energy for the CWA today into tonight ahead of that upper trough.
Low level winds should be southeasterly much of today well into
tonight. Most of the models also indicate a decent Denver cyclone,
with the convergence line east of Urban Corridor. There should be
pretty decent moisture for the CWA today into tonight looking at
upstream satellite pictures. The trough has some clouds with it.
Precipitable water values are progged in the 0.75 to 1.10 inch
range for the plains this afternoon and evening. The dew point
proggs keep values in the 50s and lower 60s F for the plains and
lower foothills through this evening. There is decent CAPE progged
for the plains this afternoon and evening with values over 2000
J/kg. Lapse rates are pretty steep on the cross sections as well.
Forecast soundings show a bit of shear too. There are decent
rainfall amounts on the QPF fields, especially over the eastern
half of the plains this evening. So with the addition of the
synoptic scale energy, will go with pops in the 20% range for the
western CWA up to 50%s over the east this afternoon and evening.
There could be some severe storms out east. For temperatures,
today`s highs look to be 1-3 C cooler than yesterday`s highs.

.LONG TERM...(Saturday through Thursday)
Issued at 319 AM MDT Fri Aug 23 2019

Warmer and drier air aloft will move over Colorado this weekend.
There will be some lingering low level moisture over the eastern
part of the plains on Saturday that will mix out more by Sunday.
At that time, 500 mb temperatures around -4 combined with the
drying should cap convection. There will be some increase in the
flow aloft in response to a shortwave over the northern plains.
700 mb winds should be around 20 knots with 600 mb winds of 30
knots. This will cause fire weather issues...see below for more on
that. Mid level temperatures will be about as warm as they get,
but the nights are getting longer, and some low clouds on the
eastern plains may slow the warming on Saturday as well. The
current forecast for Saturday and Saturday night is several
degrees above guidance but that`s probably OK. There will likely
be enough wind for a mild night Saturday night in most places,
helping it get warmer on Sunday.

There continues to be decent agreement on the large scale upper
pattern through the next week, but the details result in a large
range of solutions for the surface fields. The main issue is
details of a jet streak coming across western Canada in the early
part of the week affecting a possible reinforcement of cooler air
pushing south into the US, or not.

There`s been good agreement on the first cold front early Monday
with a 10-15 degree temperature drop east of the mountains and
some increase in low level moisture. The cooling should be enough
to keep things stable, with the possible exception of the moisture
creeping up into the mountains southwest of Denver for an
isolated storm.

From Tuesday on, there`s a quick divergence based on the possible
frontal positions and strengths. The GFS ensembles are generally
stronger with the upper level low over Ontario and about a third
of them have a stronger shortwave pushing a front down the Rockies
on Wednesday or Thursday. The operational models keep the front
well north and east of Colorado with some recovery of temperatures
in the middle of the week. There`s better agreement on another
cold front about Friday, though the strength varies. The result of
this is a big range in temperatures, with the NAEFS based MOS
having highs ranging from the lower 60s to the upper 90s on
Wednesday and Thursday. The EC based MOS has a smaller spread,
mostly in the 70s and 80s. For now, the blended solution seems
reasonable as the trend has been away from the colder solutions
during the week. The moderate temperatures and presumably some low
level moisture would justify some low PoPs, though the operational
models seem to flush out the moisture enough to be pretty dry. In
those solutions, it also stays fairly warm aloft so it may be hard
to overcome the capping unless it gets hot again. It appears the
threat of moisture from the tropical system working into the
southwest early in the week will have a hard time getting this far
north through the ridge axis and a bit of a northerly component to
the mid level flow.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 843 AM MDT Fri Aug 23 2019

Surface winds have shifted northwest at kden/kbjc and will
continue to a north and northeast by afternoon. KAPA will see a
w-nw shift between 15-16z. Another round of showers and storms
this afternoon, and like yesterday may be more of a wind impact
given the drying low levels. Best chances seem to be around 20-23z
but could see some earlier outflow winds as early as 19z and will
adjust upcoming 18Z tafs accordingly.

&&

.FIRE WEATHER...
Issued at 319 AM MDT Fri Aug 23 2019

Warmer and drier air will bring increase fire danger over the
weekend. On Saturday there will be breezy conditions in the
afternoon, mainly over the mountains, and some areas will be near
Red Flag criteria for a few hours in the afternoon. Winds will be
stronger on Sunday and humidities will still be low, so the danger
should be highest then. The main area of concern will be the
northern mountain valleys, but we may need to consider watches or
warnings for other mountain areas as well as the event draws
nearer.

For early next week, the mountains will remain warm and dry but
winds will be lighter. There may be increased winds again by
Wednesday or Thursday. East of the mountains it will be cooler
with some increase in humidities.

&&

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

$$

UPDATE...Entrekin
SHORT TERM...RJK
LONG TERM...Gimmestad
AVIATION...Entrekin
FIRE WEATHER...Gimmestad

NWS BOU Office Area Forecast Discussion