Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 151558
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
958 AM MDT Tue Aug 15 2017

.UPDATE...
Issued at 937 AM MDT Tue Aug 15 2017

No significant updates in the grids. Have included the mention of
severe storms this aftn over the far northeast plains. With the
surface low south of the Palmer Divide, easterly surface winds
should continua to advect the low level moisture further westward
towards the Urban Corridor through the day. Precipitable water in
the 1-1.2 inch range over the northeast plains, so should see pw
values gradually increase this aftn towards the Front Range. Main
threat for severe storms will continue to be the far northeast
plains, with large hail and damaging winds the main concern with
any severe storms, along with very heavy rain 1-2 inches in less
than an hour. Main cold front still progged to move southward
around 00z, with stronger storms possibly forming along the
leading edge of this front late this aftn and this evening.


&&

.SHORT TERM...(Today and tonight)
Issued at 240 AM MDT Tue Aug 15 2017

Main concern today through this evening centers around severe
weather potential over the plains of northeast Colorado.

Currently, there are still some lingering light rain showers over
the plains and some regeneration of showers occurring in the
mountains north of interstate 70. This is occurring due to some
weak qg lift from upper wave lifting northeast into Wyoming.
Earlier this evening, outflow from stronger convection to the
east has replenished low level moisture over the front range and
plains with dewpoints again around 50. Overall, Precipitable
water values are up slightly from 24 hours ago with readings just
over an inch over the plains and about 3/4 of an inch over the
Front Range.

Forecast will be impacted by upper level trof over the Great Basin
which will move across Colorado tonight and associated cold front.
There is some light to moderate QG ascent especially this evening
and overnight across the plains of northeast Colorado which will
help enhance convection by this evening. High resolution models
continue to focus best chance for stronger/severe storms over
northern sections of CWA especially north of the interstate 76
corridor with expected convergence with incoming cold front and
moist southeast low level flow over the far plains. Surface based
CAPE upwards of 2500j/kg to work with and sufficient mid level
flow of 25-30kt. Expect a few storms to become severe on the
plains this evening. Following cold frontal passage, expect an
increase in low clouds over the plains and scattered showers
through the night with continued ascent from the trof expected.
Will keep some low pops through much of tonight.

.LONG TERM...(Wednesday through Monday)
Issued at 240 AM MDT Tue Aug 15 2017

By Wednesday morning...models show the core of the upper trough
passing over eastern Colorado and with it comes a northerly low-
level flow which will lower max temps by several degs from the
day before. The past few days model temp guidance has trended
cooler on Wednesday with each run. Forty-eight hours ago, a
consensus of the temp guidance gave Denver a high of 81 on
Wednesday, and yesterday`s output a high of 79. Today`s guidance
is even cooler with a max temp of 75. This seems plausible with
the amount of cooling aloft and expected cloud cover. Meanwhile,
high country temps will also drop on Wednesday, but to a lesser
magnitude.

As the trough passes, should see PW values slipping during the
morning, only to rise again during the afternoon esply along the
Front Range with the BL flow turning into the foothills. NAM and
HiRes models show PWs nearing an inch again in this area with the
upslope flow causing CAPES to rise to around 400 j/kg over the
foothills. But with increasing cloud cover and cooler temps, it
may require most of the afternoon before we see t-storm form, and
most of this convection should be confined to the high terrain
and adjacent plains. Should see a second area of convection
across the nern corner of the CWA with lift generated by a
sharply curved 60-70kt jet rounding the bottom of the departing
trough. With CAPEs approaching 1000 j/kg out there, I put t-storm
chances at 30-40 pct from mid-morning through early afternoon,
but nothing coming close to severe. After which time, storm
chances should quickly fall off with an influx of much drier,
stable air from the north. Light showers and isolated t-storms
over and along the Front Range should also come to an end for the
same reason during the early evening hours. Overnight, should see
skies gradually clear from north to south as the upper level flow
assumes a nearly zonal component by morning.

On Thursday, models show another weak shortwave trough swinging down
from Idaho and passing over the forecast area by late in the day.
Expect to see another round of showers and t-storms, but not with
the coverage or intensity of that seen earlier in the week. More
sunshine, a south-southwest low-level flow and a 1-3 deg c warmup
aloft should return max temps on the plains to the low and mid
80s, and high country readings to the 60s and 70s.

During the period Friday through Monday, conditions appear to remain
a bit unsettled as models indicate the passage of a series of weak
disturbances in the quasi-zonal flow to start out, and in a
southwesterly flow by late in the weekend with a high pressure
ridge attempting to build over the state. Growing differences in
the models add more uncertainty to the forecast during this
period. However, the GFS, ECMWF and CMC all show a trend towards a
much wetter pattern by early next week with a very moist
southerly flow developing ahead of a deep trough moving onshore
in California. For now will gradually trend PoPs upward during
this period.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 937 AM MDT Tue Aug 15 2017

Surface cold front will impact areal terminals by this evening as
winds increase from the north. Could see isolated thunderstorms
developing around the Denver area by 21z, but the best chance of
showers and thunderstorms will be with this front from late
afternoon through the evening hours. Chance of low clouds in post
frontal regime with MVFR cigs developing late evening and
overnight.

&&

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

$$

UPDATE...Cooper
SHORT TERM...Entrekin
LONG TERM...Baker
AVIATION...Cooper



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