823
FXUS65 KGJT 190514
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1114 PM MDT Tue Jun 18 2024

.KEY MESSAGES...

- Due to a cold front highs today will be 15-20 degrees cooler
  than yesterday.

- Southwest winds increase on Wednesday with gusts 25 to 35 mph
  possible across west-central and southwest Colorado, when in
  combination with low relative humidity and dry fuels could
  lead to potential critical fire weather conditions.

- Unsettled conditions return Thursday with afternoon showers
  and thunderstorms expected into the weekend. Modest moisture
  could support the potential for heavy rain during this
  period.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 316 PM MDT Tue Jun 18 2024

Temperatures are a lot cooler today with a drier air mass as
well following passage of last evening`s cold front. The front
has cleared the area this morning with some cumulus clouds
forming over the high terrain but mostly sunny to sunny skies
otherwise. It is breezy in some areas with gusts 20 to 30 mph on
average but much less than yesterday. Southwest flow will
increase on Wednesday as a low pressure trough deepens through
California with the gradient tightening a bit across the Great
Basin into western Colorado. Some moisture gets pooled up from
the south as the high is shifted eastward, but most of this
moisture stays along the divide and east over the the Front
Range on Wednesday. This could result in some isolated to
scattered storms given steep lapse rates and a little bit of
instability along the divide. Mixing ratios are upwards of 5
g/kg along the divide but given dry low levels, expect gusty
outflow winds to be the primary concern with some localized
pockets of moderate rain possible. Much drier conditions still
exist over eastern Utah and western Colorado, leading to
potential for critical fire weather conditions across lower
elevations of west-central and southwest Colorado in particular.
With relative humidity in the single digits to low teens and
winds gusting 25 to 30 mph in these areas and potentially
higher in some areas, decided to issue a Fire Weather Watch for
Wednesday. More moisture arrives Wednesday night as the moisture
plume shifts a bit more westward with southwest Colorado seeing
some showers and thunderstorms with mixing ratios increasing
across southwest Colorado to around 7 g/kg and CAPE values
increasing 500 to around 1000 J/kg. This increase continues
into Thursday and Friday which is discussed in the Long Term.
The moisture advection will at the least increase the relative
humidity levels above critical fire weather thresholds by
Wednesday evening into Thursday.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 316 PM MDT Tue Jun 18 2024

By Thursday, the high pressure center in the East retrogrades into
the Midwest with the western flank pushing into New Mexico to pull
the tropical moisture off the Gulf of Mexico across northern Mexico
and Arizona into eastern Utah and Western Colorado. The trough along
the West Coast will keep cooler and drier air in the upper levels
over the region to support higher CAPE as the moisture moves into
the region in the lower and mid levels. IVT`s of 300 to 600 kg/m/s
move up from Arizona through the afternoon and evening indicating
the possibility for stronger showers and thunderstorms across the
region producing heavy rain. Guidance is focusing on the southern
Mountains where orographics will play to wring the moisture out of
the air, but significant moisture will make it up into the central
and northern areas overnight into Friday. Though the jet flow is on
the weak side aloft, that combined with a shortwave trough moving up
from the southwest will add support to convection across the region
Friday. By Saturday, the high pressure slides into the Desert
Southwest to shut off the tap to the moisture. With the high
pressure building in over the Desert Southwest going into next week,
temperatures will climb back to wards the century mark in the lower
valleys and generally run five to ten degrees above normal for late
June.

So for the forecast, look for showers to develop over the southern
mountains Thursday morning spreading north and east into the central
mountains and the Four Corners area through the afternoon. With
diurnal heating, showers and thunderstorms will be numerous across
the southern areas and more scattered across the central areas up to
about the I-70 corridor. Convection will settle down to widely
scattered across the entire region overnight into Friday. Heating
will be limited by the heavy cloud cover Friday, but with cooler air
aloft and weak jet support, showers and thunderstorms will become
widespread across the region through the afternoon with the heavier
rain along the southwestern faces of the higher terrain. Showers
diminish northwest to southeast overnight Friday with isolated to
scattered showers lingering through the day Saturday.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1112 PM MDT Tue Jun 18 2024

VFR conditions with prevail over the next 24 hours. Gusts 20 to
25 kts are possible across most sites by Wednesday afternoon.

&&

.FIRE WEATHER...
Issued at 316 PM MDT Tue Jun 18 2024

Conditions remain dry through Wednesday before moisture
increases across the region. On Wednesday, relative humidity
will be in the upper single digits to low teens with southwest
winds increasing compared to today. Winds will generally gust up
to 30 mph and could gust up to 35 mph in some areas of west-
central and southwest Colorado. Therefore, decided to issue a
Fire Weather Watch for Wednesday to account for this potential
where fuels are deemed critical. Deeper moisture moves across
the region Wednesday night into Thursday and Friday, bringing
relative humidity levels well above critical fire weather
thresholds with widespread showers and storms and potential for
moderate to heavy rain. The initial surge of moisture, however,
could also support an increase for lightning starts, especially
Wednesday evening into Thursday afternoon.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Fire Weather Watch from 11 AM MDT Wednesday through Wednesday
     evening for COZ203-207-290-292.
UT...None.

&&

$$

SHORT TERM...MDA
LONG TERM...DB
AVIATION...TGJT
FIRE WEATHER...MDA