LIX issues Area Forecast Discussion (AFD) at Dec 4, 10:34 AM CST ...New UPDATE...822
FXUS64 KLIX 041634
AFDLIX
Area Forecast Discussion
National Weather Service New Orleans LA
1034 AM CST Wed Dec 4 2024
...New UPDATE...
.UPDATE...
Issued at 950 AM CST Wed Dec 4 2024
Performed a gridded update this morning to bring in obs/recent
short-range guidance into tonight and extended a bit longer into
during the day on Thursday to get an early idea of what our next
system is looking like. Starting with right now, the 12Z KLIX RAOB
illustrates plenty of dry air aloft, but are beginning to see a
few moist layers introducing mid/upper-level clouds, primarily at
H7 and H3. Recent GOES-16 VIS shows these clouds, pretty thick in
coverage advancing from the west. Because of this, did drop highs
slightly cooler today as these clouds continue to build in
(towards the NBM 25th percentile). Otherwise, we'll see steadily
moistening of the column throughout the day with light upper-level
returns already being seen from KHDC to the west. This is all from
a weak upper-level impulse, with very dry antecedent tropospheric
conditions so priming/moistening will take time, likely in the
form of virga from the west to the east this afternoon/evening,
then sprinkles making it's way to the ground with time.
As far as the details and dynamics at play with this impulse
overnight, CAMs are in good agreement in what I mentioned before,
mainly stratiform isentropically forced light rain showers
advancing from the west, helping to drag a weak warm front back
north and introduce a brief/weak warm sector across I-10/12 south.
Given stronger frontogenetic/dynamic forcing approaching from the
NW, intensifies the QPF shield and orientates it more SW to NE
sliding east with time into early Thursday.
Not anticipating any hazardous weather associated with this
system, as unstable air return flow magnitude/residence time is
very minimal, however, enough dynamic forcing could support a few
elevated cells embedded in the QPF shield to produce small hail.
Otherwise, total QPF range looks confidently between 0.75 to 1.5
on average, with all activity quickly departing around sunrise.
Will dive deeper in the afternoon package. KLG
&&
.SHORT TERM...
(Today through Thursday)
Issued at 335 AM CST Wed Dec 4 2024
Longwave trough remains over about the eastern half of the country
early this morning. One shortwave moving through that trough is
exiting off the Atlantic Coast, while the next is moving through
Wisconsin and Iowa. There is a southern stream shortwave ridge
over Texas, and a larger scale ridge over the Pacific Northwest. A
southern stream shortwave was over Arizona. At the surface, high
pressure was centered near Atlanta. Water vapor imagery shows a
significant plume of moisture moving through the southern stream
from the Pacific across northern Mexico and Texas. Mid level
clouds are rapidly approaching the area. In fact, the observation
from Houma is already reporting 12,000 foot ceilings. Temperatures
at 3 AM CST ranged from 30 at Bogalusa to 52 at Lakefront Airport
in New Orleans. As the mid level clouds arrive this morning,
temperatures will probably rise several degrees.
The 00z LIX upper air sounding showed a precipitable water value
of 0.33 inches, and LCH wasn't much more moist at 0.47 inches.
With low level RH values only around 30 percent in those
soundings, it's going to take a while to saturate the lower layers
enough to produce significant precipitation today. Precipitable
water values finally reach 1 inch late this afternoon and are
forecast to top out around 1.5 inches late tonight just ahead of
the shortwave passage. While there is plenty of shear available,
instability is almost nil, so any embedded thunderstorms should
remain pretty isolated, if they occur at all. Most of any
significant rainfall should occur between midnight tonight and
noon on Thursday, with most of the area west of Interstate 55
seeing an inch or two, and areas east of that generally less than
an inch. The front should push offshore by noon Thursday, with the
main question just how far offshore the front gets.
The temperature forecast is a bit of a challenge in the short
term. Today is relatively straight forward as temperatures are
above 60 degrees just offshore of the lower Louisiana coastline.
Winds are expected to turn onshore this morning, which will bring
that air across much of the area. Temperatures are likely to
remain in the 50s overnight, as it currently appears that the
frontal passage will occur after sunrise on Thursday, but a faster
frontal passage would cut off heating, so a temperature bust on
highs Thursday isn't out of the question. In any case, it wouldn't
be much of a surprise to see highs occur prior to noon Thursday,
with temperatures falling back a bit in the afternoon. Guidance
temps are actually in comparatively good agreement through
Thursday.
&&
.LONG TERM...
(Thursday night through Tuesday)
Issued at 335 AM CST Wed Dec 4 2024
Unfortunately, forecast confidence drops off beyond the daytime
hours Thursday. The main issue for Thursday night into Saturday
will be the positioning of the front after the Thursday passage
across the area. The ECMWF is much more aggressive with the
frontal passage, and pushes it well out into the Gulf, while the
GFS operational keeps it fairly close to the coast. The ECMWF,
which is internally consistent with it's ensemble, would dry out
the area and involve much more cold air advection, especially in
northern areas. The GFS solution would hold clouds across most or
all of the area for at least Thursday night and Friday. There's
about a 10-15F difference in low temperatures between the GFS and
ECMWF forecast lows for Friday morning, with the ECMWF numbers
being the coldest of the season. Considering the surface wind
fields, and the fact that winds aren't expected to totally
decouple, the ECMWF numbers appear to be too low for lows Friday
morning. Conversely, the GFS numbers for highs Friday look to be
too warm, as it's solution would require sunshine to reach those
levels, which doesn't look to happen based on forecast soundings.
The NBM deterministic numbers are pretty much a compromise
between the ECMWF and GFS operational deterministic values.
High pressure shifts eastward to the South Carolina coast by
Saturday evening with winds turning onshore again. This will allow
the airmass to warm back up...eventually. With a fair amount of
cloud cover expected Saturday, temperatures could struggle to get
to 60 degrees. A strong southern stream shortwave is forecast to
move across the area Sunday night and Monday with more showers and
thunderstorms. With forecast precipitable water values near 1.8
inches, that is near the top of the chart climatologically for mid
December, there will be a threat of heavy rainfall. With the
ground already wet from rainfall tonight and tomorrow, a couple or
three inches of rain could cause some issues. WPC already
highlighting a Marginal to Slight Risk of excessive rainfall for
Day 5, which would be primarily Sunday night. Again, available
instability will be rather limited, so at least for now, it
doesn't appear to be a significant threat of severe weather.
Below normal temperatures anticipated for Thursday night through
Saturday, although morning lows associated with the ECMWF solution
look to be a bit too cold. Sunday through at least Tuesday will
see above normal temperatures, with highs likely in the 70s for
much of the area. A northern stream shortwave will move across the
area Tuesday night bringing colder air back to the area, but at
least for Wednesday, probably only down to normal levels. Didn't
stray far from the NBM deterministic values for now.
&&
.AVIATION...
(12Z TAFS)
Issued at 535 AM CST Wed Dec 4 2024
Mid and high level clouds moving rapidly into the area, but not
seeing much below FL120 at this point. Radar detecting echoes in
the Lafayette area, but surface observations not reporting any
rain at this point. Forecast soundings indicate it is going to
take quite a while to saturate lower layers to the going where
flight restrictions become necessary. Could be some MVFR
conditions by mid to late afternoon across the western terminals,
with widespread MVFR conditions this evening, and IFR after
midnight. Not carrying TSRA in terminals at this time, but
overnight threat is non-zero. Frontal passage likely to occur
pretty close to the end of the forecast period in western
terminals, with passage across remaining terminals between 12z-18z
Thursday.
&&
.MARINE...
Issued at 335 AM CST Wed Dec 4 2024
Winds have relaxed enough over the open waters to allow us to drop
the Small Craft Exercise Caution headlines a little early. Should
be a comparatively calm period for about the next 24 hours until
cold air advection arrives behind the front Thursday afternoon or
Thursday night. At that point we will likely need Small Craft
Advisories from Thursday night through the daytime hours Friday.
Winds then relax until warm advection returns moderate onshore
winds during the day Sunday.
&&
.PRELIMINARY POINT TEMPS/POPS...
MCB 61 49 57 28 / 60 100 30 0
BTR 68 56 64 36 / 60 100 30 0
ASD 67 53 67 34 / 30 100 60 0
MSY 68 58 67 41 / 20 100 70 10
GPT 66 55 66 34 / 20 100 70 0
PQL 68 52 68 32 / 10 90 80 0
&&
.LIX WATCHES/WARNINGS/ADVISORIES...
LA...None.
GM...None.
MS...None.
GM...None.
&&
$$
UPDATE...KLG
SHORT TERM...RW
LONG TERM....RW
AVIATION...RW
MARINE...RW
Source:
LIX issues Area Forecast Discussion (AFD) at Dec 4, 10:34 AM CST ...New UPDATE...---------------
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