788
FXUS65 KGJT 221156
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
556 AM MDT Sun Sep 22 2024

.KEY MESSAGES...

- Showers linger over western Colorado this morning with the
  northwest San Juan Mountains favored. Light snow at pass level
  could lead to slick conditions.

- Showers and embedded thunderstorms pick up again this
  afternoon, mainly over the mountains, as daytime warming
  results in instability.

- Unsettled conditions remain in place through Monday, with
  afternoon thunderstorms possible, mainly over higher terrain,
  before drier conditions move in Tuesday through the end of the
  week.

- Temperatures rebound to near normal Monday and Tuesday
  followed by a warming trend during the latter part of the week
  as high pressure builds over the region.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 335 AM MDT Sun Sep 22 2024

A vorticity lobe moving through the southwest quadrant of the
closed low pressure system centered over the central Colorado
mountains was responsible for a broad area of stratiform
precipitation encompassing much of Mesa, Montrose, San Miguel
and Ouray counties early this morning. Models indicated the low
begins to fill after sunrise shifting slightly east of the
Continental Divide by noon. Lapse rates ease as the cold pool
aloft shifts off to the east and the vort-lobe passes to the
south of forecast area. However, northwesterlies on the back
side of the low working on saturated air at lower levels is
likely to keep some scattered showers going over the northwest
San Juan Mountains, and to a lesser extent, the central Colorado
ranges. Lapse rates steepen with daytime warming so the higher
terrain will see isolated to scattered showers and thunderstorms
redevelop in the afternoon with more numerous storms over the
San Juans. As has been advertised over the past several days,
highs will be cooler than normal, though perhaps not as cool as
earlier thought.

Drying begins tonight as flow continues out of the northwest.
Skies will be clearing leading to good radiational cooling
which, in turn, is likely to aid the formation of fog in the
higher valleys. On Monday, a low amplitude mid-level short wave
trough moves southeastward over the forecast at peak heating.
Models suggest enough moisture remains to fuel isolated showers
and/or thunderstorms over the higher terrain. Increased sunshine
combined with weak warm air advection is expected to bring
temperatures to near normal levels to start the week.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 335 AM MDT Sun Sep 22 2024

The remainder of a trough of low pressure moves eastward Monday
night as an amplified center of high pressure takes over much
of the West Tuesday. With the polar jet pushed north of the high
and ample dry air in place, weather conditions should remain
dry, warm, and quiet this coming week. Winds under the high
should remain rather calm too. A few afternoon terrain based
gusts can be expected each day, but gradient winds should stay
light. Temperatures under the high will surge well into the 80`s
for our low valleys and 70`s for most of our mountain towns
this week, making it feel much more summer-like than the fall
conditions this weekend. Model agreement begins to diverge
dramatically mid week though. A series of PACNW waves along the
polar jet chip away at the blocking ridge beginning Wednesday. A
potential tropical cyclone comes into the picture in the Gulf
of Mexico too. Models are struggling mightily to resolve the
downstream impacts of this tropical storm. For now, the
flattening ridge hangs over the Rockies into the weekend with
the polar jet still parked along the Canadian Border. Afternoon
highs will feel a bit warm for late September, averaging 10
degrees over climatology. Morning lows will remain pleasant this
week thanks to lower dewpoints hanging on. Hopefully the cool
overnight conditions will balance out the warm afternoon highs
this week as we wait patiently for the arrival of the next round
of fall weather here on the West Slope.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 556 AM MDT Sun Sep 22 2024

Showers were winding down across western Colorado early this
morning. A portion of the Uncompahgre Plateau and the northwest
San Juan Mountains continued to see some light activity
according to radar. Meanwhile, to the north, showers moving
east-southeastward appeared to dissipate once the barrier was
cleared. Some low clouds lingered in the Roaring Fork Valley and
threaten to bring a period of LIFR ceilings through 15Z with
ceilings below ILS breakpoints likely to persist at KASE through
18Z. Showers and isolated thunderstorms are expected to
redevelop over the higher terrain this afternoon, but aren`t
likely to impact TAF sites and should quickly dissipate in the
early evening. Elsewhere, VFR conditions prevailed, and it`s
unlikely this will change for any of the TAF sites in eastern
Utah and western Colorado throughout the next 24 hours. Winds
will be light, and generally terrain driven.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...None.
UT...None.

&&

$$

SHORT TERM...NL
LONG TERM...LTB
AVIATION...NL