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Area forecast discussion 
National Weather Service Grand Junction Colorado
402 am MDT Friday Aug 22 2014

Short term...(today through saturday)
issued at 402 am MDT Friday Aug 22 2014

The persistent low over Southern California has begun to eject as
the jet stream around the feature now unbalanced and stronger on
the the eastern side. Latest models in good agreement with rap
analysis placing the middle-level center over west-central Arizona.
This repositioning has opened the door to subtropical moisture
which has surged back into The Four Corners region early this
morning. Divergence associated with the left exit region of a
65-70kt jet overrunning the moisture tongue appeared responsible
for the band of showers over southwest Colorado early this
morning.

Models indicated the moisture and showers will move northward this
morning as the low center fills and pushes to near The Four
Corners. The 70kt jet streak becomes oriented along a north-south
line over western Colorado and will continue to be the focus for
showers and isolated thunderstorms before noon. During the
afternoon the low lifts into western Colorado as an open wave and
the jet translates eastward becoming aligned with the Continental
Divide. As a result...divergence aloft shifts to the Front Range.
Broad lift and ubiquitous moisture will produce widespread showers
and scattered thunderstorms.

Cloud cover will significantly limit insolation during the day so
expect afternoon highs will remain relatively cool for this time
of year. Mild temperatures will also limit instability and cape so
expect precipitation will be less intense. However...precipitable water values
from 0.8 to 0.95 sufficient for broad areas of moderate rain with
pockets of heavy rain. Rain may generate mud or debris flows in
steep terrain but do not see a potential for significant flash
flooding so will not be issuing an flash Flood Advisory with this package.

Today/S upper disturbance that originated over Southern California...will
continue on its trek to the northeast...passing over the
northwest Colorado during the evening. This will keep the evening
very convectively active...especially north of Interstate 70 where
activity will peak during the early-middle evening hours. Heavy rain
will continue to be a concern...especially with soils saturated in
many locations due to recent rain. In the south conversely...the
air mass will become more stable behind the wave with showers and
thunderstorms diminishing from south to north...especially once
diurnal heating subsides.

By late tonight/early Saturday...the next system will approach.
The NAM shows hardly a break between the exiting wave...and a
northwest originating shortwave rotating around the base of a
longer-wave trough over the western states and southwest Canada.
Therefore nocturnal showers and thunderstorms will persist through
the late night hours...especially over the north. The passage of
the shortwave will be well timed to take advantage of the daytime
heating...so activity will increase and spread southward during
the day on Sat. Also by Sat afternoon the upper jet adds support
as a 70-80kt speed maximum passes over northeast Utah and northwest Colorado.
With the best energy in the north...that area will remain favored.
The stronger flow will keep storms moving along...so heavy rain
not as much of a concern. Temperatures will remain below normal
due to clouds/pcpn.

Long term...(saturday night through thursday)
issued at 402 am MDT Friday Aug 22 2014

Showers and thunderstorms will persist...before winding down middle
to late Sat evening as drier and more stable conditions move in
behind the shortwave. Models continue to show Sunday as a much
drier day than we have seen for quite awhile. Temperatures are
prognosticated to modestly increase with dewpoints dropping around 8-10 f
degrees by maximum heating Sunday. There could still be some diurnal
convection over the higher terrain...but any that does fire will
be isolated in nature.

European model (ecmwf) and gfs40 similar on Monday with next shortwave rotating
through the base of the longwave trough over the west shifting to
the Great Basin. Consequently...expecting a fairly active day
with scattered showers and thunderstorms across much of the
forecast area. However...models continued to struggle with
movement of the shortwave during the latter part of the week. GFS
suggests the trough axis passes eastward across the area Tuesday
with dry northwest flow settling over the region...veering to a
more westerly direction later in the week. In contrast...the European model (ecmwf)
was much more deliberate moving the wave from the Great Basin to a
position just east of the forecast area by late Thursday. As a
result...European model (ecmwf) solution wetter and cooler. Given model differences
will continue to try to strike a balance between the two.

&&

Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 402 am MDT Friday Aug 22 2014

A monsoonal surge will bring an increased chance of showers and
thunderstorms throughout the next 24 hours...mainly east of a Four
Corners-kaib-kmtj-kril-kcag line. As a result...expect periods of
mountain obscuration east of the line with occasional showers or
thunderstorms at taf sites along and east of the line. Flight
categories will likely fall into the MVFR range due to heavy rain.
Stronger cells may drive visibility into the IFR range with MVFR ceilings.
West of the line...showers and thunderstorms will be more
scattered with significantly less chance for Airport impacts from
thunderstorms. Showers and thunderstorms will continue overnight
with the focus shifting to the north.

&&

Hydrology...
issued at 402 am MDT Friday Aug 22 2014

Moisture will increase today as a disturbance from the southwest
moves across into area. As a result...showers have moved up from
the south into southeast Utah and southwest Colorado early this
morning. These showers will become more convective with
thunderstorms developing by middle to late morning...and will spread
northward throughout the day. By late in the day scattered to
numerous showers and thunderstorms will blanket most of the
County Warning Area...favoring the higher terrain. Some of these storms may
produce moderate to heavy rain. Due to the recent rainfall...soils
may quickly become saturated for localized flooding or mud and
debris flows on area roadways.



&&

Gjt watches/warnings/advisories...

Colorado...none.
Utah...none.

&&

$$

Short term...nl/eh
long term...eh/nl
aviation...no layers
hydrology...eh

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