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Area forecast discussion...updated
National Weather Service New Orleans la
652 am CST sun Dec 15 2019

conditions appear to have improved across the area enough to
cancel the dense fog advisory. There still may be a patch or two,
particularly along the Pearl River, but this should be short-lived
and lifted by 8 am or so. Updated zones and marine products to
remove headlines. No other changes at this time. Out shortly.


Previous discussion... /issued 336 am CST sun Dec 15 2019/

Short term...
the main concern in the short-term is the dense sea fog currently
blanketing much of the area this morning. As winds become
increasingly more onshore oriented and deeper, fog should lift and
disperse rather quickly in the mid-morning. Will maintain status
quo on dense fog advisory at this time. Temperatures should have
no problem warming into the 70s as clouds break and allow some
sunshine and warm air advection. Fog formation tonight, though
likely, may not be as extensive in coverage or density if
dewpoints continue to rise through the night to impart more of a
low cloud deck. This will have to be monitored and assessed more
closely this evening but overnight lows should be warmer than this
morning, possibly warming through the night. Meanwhile,
cyclogenesis gets underway in West Texas tonight with increasing
baroclincity inducing a warm front along the I-20 corridor later
today into tonight. This will firmly place the local forecast area
in the warm sector with instability and moisture depth steadily
increasing throughout the day Monday. A review of sref
thunderstorm parameters and local chap guidance strongly
indicating severe thunderstorm potential in the late afternoon and
early evening hours Monday, generally in the 3 PM to 9 PM window,
depending on what model is used. The NAM timing is 1-2 hours
faster than the GFS thermodynamics, but focused around the 17/00z
timeframe, +/- 1 to 2 hours. Sref guidance showing discrete
supercellular nature ahead of frontal qlcs in the late afternoon
hours during peak heating and greater instability. When assessed
in chap, this is yielding a EF2-3 tornado potential, gusts 60-70
mph, and Marble to half dollar hail. Areally, it appears this
would favor SW MS, upper Florida parishes to about the Pearl
River. Further south, though severe potential can not be ruled
out, may have to consider stable marine layer influences to cut
into instability along with loss of daytime heating after sunset
farther east when front arrives into coastal la and MS. At this
time, Storm Prediction Center is upgrading this area to enhanced for the Northshore
parishes and southern Mississippi counties away from the coast,
and slight for the coastal and South Shore parishes/counties.

Long term...
strong cold air advection pushes cold front through much of the
area by midnight Monday night, and clearly east of the MS coast
and coastal waters by daybreak Tuesday. Daytime highs will likely
occur around midnight or during the overnight, then cooling
considerably throughout the day with some more interior locations
struggling to stay in or reach the 50s. Combined with moderate and
gusty northwest winds, wind chills will make it feel like a raw day.
Precipitation ends pretty early Tuesday to preclude any
considerations for winter weather. Tuesday night lows fall into
light freeze for interior areas away from water influences, and
generally mid 30s elsewhere for a cold night under clear skies.
Wednesday is expected to be a sunny day but with cold air
advection still underway, temperatures only warm into the 50s but
winds should slacken by later in the day, leading to a
radiationally cold night. Interior lows once again in light
freeze territory for Thursday morning and mid 30s south of the
tidal lakes. High pressure Shallows out and moderates during the
day Thursday to onset a gradual warming trend into Friday. GFS is
showing another frontal system moving through late Friday. European model (ecmwf)
is slightly different in showing cold pool impulse digging into
base of amplifying trough Friday night but lacking precipitation
pattern typical of cold frontal passage. Any attendant moisture is
shown diverging upon arrival. At this time, will show lower end
pops but maintaining a stretch of cold days into the weekend.

IFR conditions in dense fog this morning likely to lift around
15z, becoming VFR by 18z. Veering winds to S-southeast will be steadily
increasing moisture in the column to allow for some fog formation
once again generally after 03z, but lower confidence on density as
marine layer low cloud deck may result, or initially start as fog
and lift to a low ceiling with warm air advection well above the
nearby water temperatures. Still beyond the valid taf timeframe,
the area will be under a slight to enhanced risk for severe
thunderstorms that may impact the terminals generally in the
16/21z to 17/03z timeframe, to be indicated in subsequent taf
issuances later today/tonight.

slack winds this morning will allow sea fog to linger near shore
but should be able to lift and dissipate later this morning as
winds continue to veer to solid onshore orientation. Moderate
onshore flow to be maintained into Monday ahead of approaching
strong cold front. This front should move into the coastal waters
west of the MS river by midnight Monday night and before daybreak
Tuesday in the MS coastal waters. Cold high pressure building into
the Gulf should place the area in solid Small Craft Advisory
criteria from Tuesday morning through Wednesday afternoon.

Decision support...
dss code: yellow
deployed: none.
Activation: none.
Activities: dense fog advisory
forecast support for the city of New Orleans
assessing severe thunderstorm potential for Monday

Decision support services (dss) code legend
green = no weather impacts that require action.
Blue = long-fused watch, warning, or advisory in effect or high
visibility event; marginal risk severe or excessive rain.
Yellow = heightened impacts with short-fused watch, warning or
advisory issuances; radar support for slight risk severe
or excessive rain.
Orange = high impacts; enhanced risk severe; nearby tropical events;
hazmat or other large episodes.
Red = full engagement for moderate to high risk of severe and/or
excessive rainfall, or direct tropical threats; events of
National significance.


Preliminary point temps/pops...
mcb 74 61 75 44 / 10 40 90 100
btr 78 63 78 43 / 10 40 80 90
asd 76 58 78 45 / 0 30 60 90
msy 76 64 78 49 / 0 30 50 90
gpt 71 61 73 48 / 0 20 50 90
pql 75 60 77 48 / 10 20 40 80


Lix watches/warnings/advisories...

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