Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 200230

Area Forecast Discussion
National Weather Service Denver/Boulder CO
830 PM MDT Wed Jul 19 2017

Issued at 830 PM MDT Wed Jul 19 2017

Bumped up PoPs across Weld, Morgan, and Logan Counties given
existing convection and trends on GOES-16 VIS/IR and area radars.
Expect the convection to dissipate or move our of our area by
midnight and current PoPs reflect that trend.

Focus on the short term continues to be on the rain chances
tomorrow. Latest model guidance indicates better low-level
moisture Thursday afternoon, more instability, and consequently
better chances of thunderstorms compared to Wednesday. Overall
confidence in the mountains and NE Plains (especially north of
I-70) getting some convective precipitation has increased with
the latest data. Diurnal easterly upslope flow should favor
convection initiating over the Front Range Mountains mid afternoon
then moving ENE across the Plains early evening. Strong, gusty
winds should accompany any storms that get going and given PWATs
in the 1-1.2" range, brief heavy rain is also a possibility.
Current forecast is in line with this potential scenario.


.SHORT TERM...(This evening through Thursday)
Issued at 334 PM MDT Wed Jul 19 2017

The upper ridge aloft remains over the south central plains with
weak southerly flow aloft in place across Colorado. Slow moving
thunderstorms have developed over the mountain areas, with locally
heavy rain occurring. Outflow boundaries from these storms may
still generate some thunderstorms on the adjacent plains through
the evening. GPS precipitable water values are in the 0.70 to 0.80
range, so the threat of heavy rain is reduced from some of the
past days.

Overnight, showers will dissipate and skies will clear out. On
Thursday, we will be keeping an eye on a circulation moving out of
west Texas this afternoon. This circulation will be moving over
Colorado tomorrow, and will affect the development of showers
through the day. Monsoonal moisture will be present, so it is
possible that thunderstorms will be capable of producing heavy
rainfall. The situation will be similar to the past several
afternoons. Model QPF fields are not producing especially large
amounts of rainfall, so it won`t be necessary for any highlights
up to this point. The threat will need to be watched out for.

.LONG TERM...(Thursday night through Wednesday)
Issued at 334 PM MDT Wed Jul 19 2017

A shortwave moving across the top of the ridge will stream across
the northern area Thursday night. This will keep scattered storms
across the area that evening decreasing after midnight. With PW
values of 1.1 to 1.5 across the northern plains, and CAPE values
around 1000 J/kg near the state border, will likely see a few
strong to severe strength storms with heavy rain, strong outflow
winds and hail. PW values of 0.75-1.0 inches over the mountains
will keep the threat of heavy rain in the evening.

Confidence in the effects of this upcoming monsoonal moisture
plume is wavering due to models performance. The GFS seems too wet
with its precipitation in regard to it specific humidity and PW
fields, where the NAM seems too dry with respect to its fields.
But overall, the specific humidity is slightly lower than what was
being shown yesterday. Still confident we`ll see heavier rain and
a greater spatial coverage of storms. On Friday, we`ll have a
front pushing down as well from the north to help provide a focus.
PW values of 1.3 to 1.5 inches and CAPE of 1000 to 2000 will allow
for strong to possible severe strength storms again, with brief
heavy rain and strong outflow again with some hail possible.
Mountains will again be under the threat of localized flooding
under stronger storms. Temperatures will be cooler by a few
degrees from increased cloud cover.

Saturday and Sunday will cool back to seasonal normals as a couple
shortwaves push the ridge south. The main moisture plume sinks
south as well, carrying with it the heavier rain threat. Still
expecting scattered showers and storms across the area, especially
south of I-76 and I-70 and down near the Palmer Divide.

Monday through Wednesday should feature a return to warmer
temperatures as the ridge builds back north. Moisture will build
back in to the area, though not as much as we`ll see Thursday and
Friday. This should result in better coverage of storms in the
mountains and a slight chance over the plains.


.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 830 PM MDT Wed Jul 19 2017

VFR conditions expected through Thursday. Light rain showers over
APA will remain southwest of DEN and south of BJC, and continue
for another hour before dissipating. The batch of showers
generated a gust front that moved across all three TAF sites
earlier this evening, leaving SSW winds in its wake. The moderate
SSW is in the process of weakening and by 03Z drainage winds
should take over. Those will continue all night and into Thursday
morning and be in the 8-12 kt range. Thursday around 17Z the
winds should come around to the east or southeast at the TAF sites
and remain on the light side (8-12 kts) until thunderstorms form
over mountains to the West. Expect convective development to
generate numerous gust fronts much like Wednesday that would
impact the terminals around 22Z or so, with wind gusts to 35 kts.
The chance of TS at the terminals Thursday evening is around 30
percent so have handled the potential of TSRA with tempo groups
for now and will update as needed as the convective evolution
becomes more clear on Thursday.


Issued at 334 PM MDT Wed Jul 19 2017

An easterly wave moving over southwestern Texas this afternoon
will be moving over Colorado through the day Thursday. This will
generate showers and thunderstorms from late morning onward. High
moisture levels and slow general storm motion will result in
locally heavy rainfall Thursday afternoon.

With monsoonal moisture pushing into the region, and moderate
storm movements, stronger storms on Thursday night will be able to
produce rainfall rates of 1/2" in 30 minutes or less. Higher
moisture levels and slower speeds on Friday will allow storms to
be able to produce rainfall rates of 3/4" in 30 minutes or less.




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