Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 270623

Area Forecast Discussion
National Weather Service Grand Junction CO
1223 AM MDT Tue Jul 27 2021

.SHORT TERM...(Tonight through Tuesday night)
Issued at 104 PM MDT Mon Jul 26 2021

PWAT on the sounding last evening was the lowest it has been in a
few days, but is still riding close the 90th percentile for late
July. Surface dewpoints were in the low 60s at the office this
morning, which is particularly modest. Models show this slightly
anomalous moisture remaining in place until further notice thus
the threat of afternoon storms with heavy rainfall continues. The
area most probable for runoff threats will continue to be across
the plateau and canyon regions of southwest Colorado and Southeast
Utah. This will be where the axis of high PWAT values will exist
and where there will still be a weakness in the atmosphere aloft.
Winds aloft remain weak so slow storm motion is a threat along
with storms forming on outflow boundaries or ahead of embedded
mesoscale features such as those last night. Several runs of the
latest high-res models show that one of these mesoscale features
develops over the northwest San Juans and moves to the northwest
and perhaps something in the central mountains that moves
westward. Localized flooding will once again be the primary threat
with burn scars and flashy terrain the most vulnerable. If you
plan on outdoor recreation today be prepared for quickly changing
conditons and the monsoonal threats of heavy rainfall and runoff
especially if you`re located in a watch.

By tomorrow high pressure will have moved out of the northern Great
Basin into the Central Plains, and will bring somewhat of a tougher
atmosphere to pop stronger storms..including our northeast CWA.
Given the placement of the high storm coverage will increase from
east to west. Storm motion will be more from the south-southeast,
which is not a prevailing direction and could result in atypical
evolution off the higher terrain. There is very little change in
the moisture so localized flash flooding will again be possible
over many of the same areas as the last few days.

.LONG TERM...(Wednesday through Monday)
Issued at 104 PM MDT Mon Jul 26 2021

The high`s center will settle over the Central and Southern Plains
on Wednesday, resulting in a surge of monsoonal moisture into our
forecast area by late afternoon. Even though we will still be
positioned underneath the ridge, this additional moisture push will
lead to numerous showers and thunderstorms across the region.
PWATs will increase to 150 to 200% of normal in response with Bufkit
soundings continuing to show a well saturated profile. Light
steering winds will exacerbate the potential for flooding as heavy
rain will be a threat with the stronger storms. Similar conditions
are expected through the end of the week as the high wobbles
slightly to the east. PWATs look to increase even more during this
time as a remnant tropical low pressure from Mexico looks to be
advected in the flow. Regardless, the atmosphere will remain primed
for heavy rain and anyone planning to travel should stay up-to-date
on the latest forecast and road conditions before heading out.

The high will begin to track southwest as a Pacific midlevel trough
lifts across the northwestern CONUS during the weekend. The base of
this shortwave will brush our CWA early next week, providing
additional lift for showers and storms. Beyond that, drier air looks
to push into the region. Until that time, cool and unsettled
conditions will prevail.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1218 AM MDT Tue Jul 27 2021

A thunderstorm in vicinity of KCNY will last though 08Z leaving
dissipating showers and high cloudiness to drift over the
remainder of the CWA. VFR conditions will prevail over the next
24 hours with another round of thunderstorms impacting the 4
Corners up to I-70 corridor tomorrow afternoon and evening.


Issued at 104 PM MDT Mon Jul 26 2021

Monsoonal moisture will remain entrenched over the area until
further notice. This means the threat of heavy rainfall will
remain a threat especially under stronger storms. Slow storm
motion and terrain interactions will also invite the threat of
excess runoff from storms and localized flash flooding will remain
a threat over much of the upcoming week. If recreating outdoors
it will be important to stay up to date on the latest forecasts
and know the threats during this active monsoon season.

A Flash Flood Watch was issued for the La Sal and Abajo Mountains
as well as portions of west-central Colorado until 10 PM MDT this
evening. We will continue to monitor the need for additional




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