Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 181025

Area Forecast Discussion
National Weather Service Denver/Boulder CO
425 AM MDT THU AUG 18 2016

.SHORT TERM...(Today and tonight)
Issued at 425 AM MDT Thu Aug 18 2016

Today looks like it will be the last of the warm days for the
week. The upper air pattern over the state is a weak and
disorganized ridge that will be giving way to the influence of an
upper trough moving out of the Pacific Northwest in the next
couple days. At lower levels, surface high pressure will be
building over Wyoming and spreading northerly flow across
northeast Colorado. The initial wind shift is expected to arrive
in Denver by midday and then continue through tonight. The airmass
will be starting out dry and stable, as evidenced by precipitable
water values around 0.70 inches this morning. By tonight, model
sounding show the PW climbing to around an inch as cooler, moist
air moves into the region. Model soundings show only weak
instability this afternoon, around 500-1000 j/kg, so only isolated
showers or thunderstorms are expected through this evening.
Mountain areas may see a little more showers due to heating over
the higher terrain.

Temperatures today should be a few degrees cooler than yesterday
due to the cool air filtering into the region. However, mostly
sunny skies should still be able to lift temperatures into the
mid and upper 80s across the plains. Most of the effects of the
approaching cooler weather are not expected until tomorrow and

Shower activity may be prolonged this evening as the upslope flow
moistens the airmass and a weak upper jet moves over southern
Wyoming and Nebraska, placing Colorado under the right entrance
region of the jet. By later tonight, the airmass may have cooled
enough that continuing shower activity will not have any thunder
in it, but it can`t be ruled out entirely.

.LONG TERM...(Friday through Wednesday)
Issued at 425 AM MDT Thu Aug 18 2016

An upper level trough of low pressure moving across the northern
Rockies will brush Colorado on Friday. A cold front associated with
the trough will move across northeastern Colorado overnight. Upslope
flow behind the front will likely produce low clouds from the lower
foothills eastward across the Palmer Divide and eastern plains. The
clouds should linger through late morning before breaking up.
Scattered showers and thunderstorms may develop across the CWA
during the afternoon hours and continue through the evening. The GFS
shows a nice batch of precipitation moving out of the northern
foothills into the Front Range Urban Corridor by 00z Saturday. The
clouds and/or precipitation should keep max temperatures on the
plains in the low 70s. The GFS sounding for Denver at 21z Friday has
a cape around 1000 j/kg and PW`s rising to 1.10 by 00z.
The sounding also indicates some shear with northeast winds from the
surface to 700mb winds and westerly winds above that. If this
verifies...we could see some strong storms with locally heavy rain
and hail moving southeast across the urban corridor during the
afternoon hours. SPC does have much of northeastern Colorado in a
marginal risk of severe thunderstorms in their day 3 outlook. The
HiRes-ARW-West and HiRes NMM-West models also show bullseyes of
heavy precipitation across portions of the Front Range
Foothills...Denver Metro Area and Palmer Divide by 00z Saturday.

Despite the optimistic outlook by the GFS as well as the HiRes-ARW-
West and HiRes NMM-West models...there are several negative factors
which may inhibit shower and thunderstorm activity.

First of all...the position of the upper trough has trended further
to the north over the past couple of well as becoming

Second...The upper level jet at 00z Saturday is located over
Wyoming (GFS and ECMWF southern Wyoming) while the NAM has it
located over northeastern Wyoming. This position may be a little too
far north...especially the NAM for a good precipitation event.

Third...the cool and cloudy weather in the morning may be slow to
break up limiting the instability. If this happens...showers and
storms may not develop until after sunset or maybe not at all.

Fourth...The NAM and ECMWF do show precipitation across the CWA
during the afternoon and evening hours...however the amounts are
much lighter than the GFS and HiRes models.

Fifth...The NAM...GFS and ECMWF all show downward QG lift at 700 and
500 MB across much of northeastern Colorado.

Due to the uncertainty...have decided not to raise pops for Friday
and Friday night. I think the scattered to high scattered pops
across the plains and Palmer Divide seem reasonable.

On Saturday...the upper trough if forecasted to move into the
Central Plains States...with a drier northwest flow aloft over
Colorado. It will be another cool day for this time of year with
some morning low clouds and fog possible across the plains and
Palmer Divide. There may also be isolated to scattered showers and
storms mainly across the mountains and Palmer Divide.

On Sunday...Upper level high pressure rebuilds over the Rocky
Mountain Region...which should result in warmer and drier weather.
Only isolated afternoon and evening...showers and thunderstorms will
be possible across the higher terrain.

On Monday...the GFS and ECMWF show the upper high shifting eastward
into the Central Plains States an upper level trough of low pressure
moves into Colorado from the west. Models hint at some monsoonal
moisture flowing into colorado from the south as well. This may
result in an increase in precipitation chances...especially across
the high country.

The upper trough is forecasted to move across the region Tuesday
through Thursday...with a possible cold front moving across
northeastern Colorado by the middle of next week. This pattern would
bring cooler and unsettled weather to north central and northeastern


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 425 AM MDT Thu Aug 18 2016

No aviation impacts expected this morning as the airmass is dry
at low levels and weak pressure gradients are spread across the
state. Generally drainage winds this morning will give way to
northerly winds by late morning and early afternoon as the initial
surge of cooler air moves across the Denver area. Daytime heating
and a moistening airmass through the afternoon will lead to
afternoon and evening thunderstorms. Moist northerly flow
continuing through tonight will cause ceilings to drop. vfr and
mvfr ceilings should be prevalent friday morning along with
scattered rain showers  or areas of drizzle.


.BOU Watches/Warnings/Advisories...


SHORT TERM...Dankers
LONG TERM...Kalina
AVIATION...Dankers is the U.S. government's official web portal to all federal, state and local government web resources and services.