1/10-1/12 Southern Slider

[fountainguy97]

3 minutes ago, John1122 said:

Titanic dry slot on the NAM 3k. Mixing. Mountain rain issues etc. The works basically. All the hi-res American models are having those issues now.  3k is bad for anywhere in East Tennessee, especially so East of 81 or south of 40.

It is a tick better and ultimately didn't get worse. Consider that  trend stopped. 
 

It would be so ironic that I've spent almost all week in the low 20s and the few hours of the week I need to be frozen I sit in the mid 30s haha 

[TellicoWx]

Upgrade to warning OHX.... 3-6

[John1122]

3 minutes ago, fountainguy97 said:

It is a tick better and ultimately didn't get worse. Consider that  trend stopped. 
 

It would be so ironic that I've spent almost all week in the low 20s and the few hours of the week I need to be frozen I sit in the mid 30s haha 

It's frustrating, for sure. The 12k NAM is super QPF heavy for everyone almost. 1 inch of precip in Knox, about .5 is snow, .3 is freezing rain, .2 is sleet and there's a little rain in there too.

[BlunderStorm]

The NAM seems determined to transform the slider into an app runner before transferring to the atlantic running in conflict with the globals and other mid to short range model solutions. Appreciable ice accumulation would be a concern through effectively all of the eastern valley, especially those counties flanking the blue ridge. It would be a hell of a coup no one is rooting for. Echoing fountainguy it does seem somewhat more favorable for the east from the added moisture though central and western TN see a modest decline in snowfall (yall are looking great regardless rn).

[John1122]

06z suite so far.

HRRR almost no freezing rain, issues along the GA border in the East. Everyone from the Plateau/Eastern Rim gets some totals cut by sleet.  SE areas are dry slotted and have less QPF overall. Even with the sleet 40 and north gets 5-8 inches with N Miss and N Bama doing well in the mid and west areas.

12k NAM a slightly worse version of the HRRR. Big time ZR in the central valley of East Tennessee

3k NAM all the problems of the 12k but magnified. Big dry slot, mixing issues for all. East of 75, especially east of 81 and south of 40 in Eastern areas aren't great. 1-2 inches of snow. .15 to .25 freezing rain in those areas. ZR is an issue south of 40 all the way to Nashville. But Plateau and West still manage 3-6 inches of snow.

[TellicoWx]

Birmingham pulls trigger on Warning..1-3

[John1122]

12z Euro Ai Copy/Paste of every run the last 5 days. .4 to .7 QPF, area wide sub-freezing temps and 850s.

[bearman]

Looks like Nashville pulled the Winter Storm Warning East should follow shortly.

arning.

[Runman292]

MRX is holding off on the warning upgrade. Leaning heavily towards the NAM while acknowledging that it's an outlier.

[bearman]

5 minutes ago, Runman292 said:

MRX is holding off on the warning upgrade. Leaning heavily towards the NAM while acknowledging that it's an outlier.

Typical though I would say under this situation I kind of surprises me.

[Runman292]

MRX AFD:

Discussion:

Bottom line up front: No change will be made to the Winter Storm
Watch at this time, due to the lowered confidence in p-types and
snow amounts. To issue a Warning beyond 24 hours, confidence must be
high that Warning criteria will be met and that impacts will be
significant.

Model trends and differences:

The NAM has thrown a wrench in the forecast with its depiction of a
warm nose spreading well northward into our area, due to its strong
southerly low level jet. This potential for mixed p-types adds much
uncertainty to the forecast snow amounts in southern sections and in
the lee of the mountains. The main difference in the NAM is that it
develops a secondary surface low in northern AL on Friday afternoon,
which tracks NE along the Cumberland Plateau to WV through the
evening. This creates a strong pressure gradient across the TN
mountains, and a 40-50 kt LLJ. However, it is noteworthy that this
is not a typical mountain wave/downslope enhancement pattern, which
is usually characterized by a 850 mb jet max to our NW, so perhaps
the NAM is overdone with the downslope warming. Also, the NAM is an
outlier in its solution, as the GFS and ECMWF do not develop this
secondary low and LLJ, and keep warm advection to our south. The
HRRR seems to be a compromise solution, and is not as aggressive
with the warm nose and LLJ as the NAM.

Precipitation types:

At onset Friday morning, temperatures profiles will support all snow
as top-down moistening evaporatively cools a dry low level layer.
HREF p-type probs show a snow/sleet/fzra mix entering southern
sections between 1 PM and 4 PM EST. The northward extent of this mix
is a big uncertainty, but for this forecast, the HREF probs look
reasonable, and keep the mix mainly to the south of I-40. The
strength of the downslope wind will affect the p-type in the
foothills north of I-40 and the Tri-Cities, and the potential for a
brief period of mixed p-types may result in lowered snow amounts
there.

Snow amounts and confidence:

The area of highest confidence in the snow amount forecast is north
of I-40 and west of I-81, in the northern Cumberland Plateau area
and KY border counties, where the p-type is highly likely to remain
all snow. In this area, amounts are expected to be in the 4-6"
range. As we go south and east from there, confidence lowers due to
the above mention p-type issues. In the southern Valley and SW NC, 1-
3 inches of snow is expected, mainly during Friday morning and early
afternoon before the mix takes over, with the potential for up to a
tenth of an inch of ice accumulation in the mid/late afternoon and
evening. Another factor in the snow amounts will be the loss of
moisture in the dendritic growth zone from the dry slot Friday
evening, which may shut off accumulating snowfall sooner than
previously forecast. The chance of accumulating precip drops rapidly
between 7 PM and midnight EST as the dry slot moves overhead.
Freezing drizzle may be a concern during this period.

Timing:

Not much has changed from previous forecasts regarding the timing of
snow onset Friday morning. Expect that the morning commute in the
Chattanooga area could be impacted by snow, potentially starting as
early as 7 AM EST but accumulating mainly in the 9-11 AM time frame
as snowfall rates increase. In Knoxville, 10 AM to noon appears to
be the likely time frame for snow to start accumulating, with Tri-
Cities being in the noon to 2 PM window. Peak precip intensity
appears to be during the afternoon, from around 1 PM EST to 7 PM
EST. Precip will begin to end near or after 7 PM from SW to NE as
the dry slot moves across the area and brings drying in the dendrite
growth zone. This end time is faster than previously forecast, but
the end time of the Winter Storm Watch will be kept at 7 AM EST
Saturday. We transition to a NW flow pattern Friday night, which may
result in some additional light snow accumulation in SW VA and the
TN mountains into Saturday morning.

[John1122]

RGEM cold, but dryish so far. To get the juice of the NAM, you have to have the insane secondary low that develops and is a plateau runner (literally never even seen that before in all my years of model viewing)

[Runman292]

It's almost as if they saw the recent runs and were like "Yes, we have an excuse not to issue warnings!" 

[John1122]

RGEM ended slightly better than 0z.  Much better thermals and no crazy secondary low traveling from Alabama to WVA.

[John1122]

ICON looks fairly similar to the RGEM so far.

[John1122]

ICON was a better version of the RGEM.

So far it's HRRR/RGEM/ICON vs NAM 12k/NAM 3k.

[bearman]

6 minutes ago, Runman292 said:

It's almost as if they saw the recent runs and were like "Yes, we have an excuse not to issue warnings!" 

Yeh the Discussion was rather frustrating.  It just seems we can never have a clear path to all snow and our office always seems to hug the outliners.  Often rightly I suppose.

[Runman292]

2 minutes ago, John1122 said:

ICON was a better version of the RGEM.

So far it's HRRR/RGEM/ICON vs NAM 12k/NAM 3k.

Maybe I've missed it, but it seems like MRX never mentions models like the ICON. Why is that?

[TellicoWx]

To me MRX broke the cardinal rule...if you have a drastic outlier that has zero support...simple toss it. You don't include it in your package or totals. Better hope they are close to right, or once again going to look like fools to general public. Especially south of 40. As far as the DGZ, that's been showing for several days now. 

[John1122]

Just now, Runman292 said:

Maybe I've missed it, but it seems like MRX never mentions models like the ICON. Why is that?

They generally only ever use the American models and the Euro. Never really the Canadian/ICON etc.

[TellicoWx]

Just now, Runman292 said:

Maybe I've missed it, but it seems like MRX never mentions models like the ICON. Why is that?

NWS don't really use it in weighting a forecast. I give it 10% weight with my own formula.

[John1122]

Just now, TellicoWx said:

To me MRX broke the cardinal rule...if you have a drastic outlier that has zero support...simple toss it. You don't include it in your package or totals. Better hope they are close to right, or once again going to look like fools to general public. Especially south of 40. As far as the DGZ, that's been showing for several days now. 

The biggest thing to me is, even those are massively impactful winter storms because they have so much freezing rain/sleet/snow. It's a winter storm for most regardless.  Heck, they even said my area was basically good for 4-6 inches regardless and didn't put the warning out here.

[BlunderStorm]

I actually like the 6z RGEM solution, no one walks away with the jackpot but everyone in or adjacent to Tennessee walks home with something appreciable.

The ICON tells a similar story but with a Bristol Bullseye and generally higher QPF!

[TellicoWx]

2 minutes ago, John1122 said:

The biggest thing to me is, even those are massively impactful winter storms because they have so much freezing rain/sleet/snow. It's a winter storm for most regardless.  Heck, they even said my area was basically good for 4-6 inches regardless and didn't put the warning out here.

I've always joking said they have a bias...but that AFD, they basically openly admitted it for first time I've can remember.

[Runman292]

3 minutes ago, John1122 said:

The biggest thing to me is, even those are massively impactful winter storms because they have so much freezing rain/sleet/snow. It's a winter storm for most regardless.  Heck, they even said my area was basically good for 4-6 inches regardless and didn't put the warning out here.

If you just read the bottom line, you would think that the storm is more of an inconvenience than a hazardous event.

[John1122]

I just generally have very little faith in the NAM beyond 24 hours, even less so when it's throwing out something pretty wild that no other model is showing. In the end, they basically have until around 7pm tonight to make a decision, as that is 12 hours before  they think this starts in Chattanooga.  I'd expect they'll make a decision after the 12z runs come in.

[John1122]

11 minutes ago, TellicoWx said:

NWS don't really use it in weighting a forecast. I give it 10% weight with my own formula.

It's actually a pretty decent model for these set ups. It's got a higher resolution than the GFS at 13k, not dissimilar to the NAM 12k

[BlunderStorm]

6z GFS rolling

[TellicoWx]

1 minute ago, John1122 said:

It's actually a pretty decent model for these set ups. It's got a higher resolution than the GFS at 13k, not dissimilar to the NAM 12k

I usually don't weigh anything until 72 hrs out...beyond that, it's more just looking at 500 trends. 72-36...I use 35% Euro, 35% GFS, 10% CMC/Ukie/ICON..everyone gets a piece of the puzzle. 36-12..NAM replaces GFS, RGEM replaces Euro...10% GFS/Euro/ICON. Under 12...Nam 3k, HRRR, RGEM, RAP...equal weight.

[TellicoWx]

And anytime you get within 36, and one goes out on an island (regardless of it hurts or helps mby or idea)...toss. and once either the others start moving toward it or it reverses...and it back to the rotation.