Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 181134

Area Forecast Discussion
National Weather Service Grand Junction CO
534 AM MDT Wed Jul 18 2018

.SHORT TERM...(Today through Thursday)
Issued at 244 AM MDT Wed Jul 18 2018

Shower activity has diminished overnight leaving scattered mid and
high level debris clouds over much of the Four Corners region.
Aloft, an elongated area of high pressure stretches from the
California Coast eastward through southern Colorado and into the
southern Great Plains. This area of high pressure will drift ever
so slightly westward on Wednesday, allowing for a slight influence
of some weak northwesterly flow over central and northern Colorado
and Utah. This will usher in a slightly drier airmass with lower
PWATs over these areas, and as a result...little to no convective
precipitation potential during the afternoon. Southern Utah and
southern Colorado will remain active with some showers and
thunderstorms over the mountains once again, however even here
activity will be weaker and less widespread than was seen on
Tuesday afternoon. While isolated heavy downpours will be
possible, the threat of flash flooding is much less than was seen
on Tuesday and no flooding highlights have been issued for
Wednesday. High temperatures will nudge upward on Wednesday as a
result of the drier airmass and less cloud cover. Have hedged
toward the higher end of guidance for the temperature forecast in
this package.

The ridge over the southwest CONUS will weaken slightly on
Thursday and remain slightly west of the Four Corners. Once again,
convective activity will be limited to mainly the southern
mountains of Utah and Colorado. Current model trends indicate that
Thursday will likely be the driest day of the forecast period, and
the threat of Flash Flooding will again be low during the
afternoon hours. Temperatures will peak on Thursday, with highs in
the triple digits likely in the lower valleys across western
Colorado and eastern Utah. Have nudged highs upward to reflect
these trends.

.LONG TERM...(Thursday night through Tuesday)
Issued at 244 AM MDT Wed Jul 18 2018

The ridge of high pressure will shift back to the east, centered
over the Texas-New Mexico border Friday through Sunday, keeping it
in prime position to tap into sub-tropical moisture from the south.
Precipitable water (PW) values increase across the Four Corners on
Friday upwards of an inch or more, with this deeper moisture
spreading northward by the weekend with an inch or greater values
across much of the forecast area. This increase in moisture will
also help increase shower and thunderstorm coverage and chances,
with the Saturday into Sunday timeframe looking especially active.
Storms also have a better chance of continuing through the overnight
hours Saturday evening into Sunday morning as a few shortwaves move
through the flow. The high pressure center looks to shift back
westward and elongate by early next week, with the GFS indicating
drier air moving back in from the northwest Tuesday, indicating a
break in the storm activity. Not quite biting off on this as
models tend to be too quick to dry things out. With the ECMWF
hanging onto the moisture a bit longer through early next week,
inclined to think that we will keep enough moisture around under
the high to keep thunderstorms in the forecast. Temperatures will
be hot through Friday with near triple digit heat in some valleys
with somewhat of a cooldown this weekend due to increased showers
and thunderstorm activity.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 534 AM MDT Wed Jul 18 2018

Drier air will continue to move into eastern Utah and western
Colorado today as high pressure slides overhead. This will result
in VFR conditions at all TAF sites as heavier cloud cover
diminishes and winds generally become light and variable. However,
some breezy conditions will occur after 18Z with gusts to 20
knots possible. Isolated thunderstorms are possible over the
southern mountains this afternoon through 02Z Thursday.




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