Clem282340
Member
Is that good or bad
-
Hello, please take a minute to check out our awesome content, contributed by the wonderful members of our community. We hope you'll add your own thoughts and opinions by making a free account!
Wintry February 19-21, 2020 Winter Storm
- Thread starter Six Mile Wx
- Start date
D
Deleted member 1449
Guest
6Z ICON has increased qpf as well, especially over the western Piedmont. Kudos to the NAM, which has really led the way with this one.
packfan98
Moderator
6z RGEM is looking solid too.
Anybody havig trouble with Pivotal? I was able to briefly get on but it then locked. The NAM did look good for the two frames I saw.
packfan98
Moderator
Well, the NWS Raleigh discussion is discouraging to say the least. I will post the full discussion here because of the important information and things to take into considuration.
4:10am
What has changed: Reduced snowfall amounts a bit across the
southeastern zones and focused the peak accumulation amounts (1-2")
a bit further north and west to better coincide with latest model
trends. Our forecast remains much lower than most raw forecast
guidance due to numerous limiting factors, including; (1) Warm
surface temperatures, (2) Self-limiting CAA processes, and (3) Low
rain to snow liquid ratios. Impacts should remain rather minimal,
however, a few slick spots may form on area roadways during the
overnight hours Thursday into early Friday morning, primarily along
and east of US-1. If the forecast holds true, a Winter Storm
Advisory issuance may be needed in future forecast packages (Our
criteria is areas that are expecting to see 1" of snow or greater in
a 12 hour period).
Full Discussion: The surface front will settle in a nearly west to
east orientation in vicinity of the SC/GA border, with ongoing
surface CAA processes occurring north of the boundary. The front is
progged to buckle back north as a series of weak disturbances begin
to develop during the day Thursday, with strong coastal cyclogenesis
beginning to initialize off the SC coast as early as Thursday
evening. Around the same time, strong Canadian surface high pressure
will continue to settle southeast through the central Corn Belt
states, extending east through the Great Lakes and northeastern US
states before beginning to south along the lee side of the
Appalachians.
Location and strength of the developing coastal low/trough will be
key to the forecast going forward, as a stronger and closer to shore
area of low pressure (NAM) will help to promote more rapid cold air
entrainment from the north. Forecast models continue to trend
increasingly more wet during the daytime hours Thursday, especially
from the US-64 corridor south during the afternoon and late evening
hours. Around this time, a 925mb cold front (marking the leading
edge of the cold nose) will extend south into the area. This will
aid, at least initially, in the ability for wet bulb/evaporative
cooling processes to promote a light rain to snow changeover to
occur in vicinity of the VA/NC border mid to late afternoon. From
there, a gradual changeover to a rain/snow mix to eventually all
snow will occur. At first, accumulations will remain little to none
with more of a "white rain" type scenario unfolding. Eventually,
especially areas east of US-1 where rain rates will remain higher
for longer, the wet-bulb effect will help to cool surface
temperatures below freezing, helping to promote accumulations
primarily on grassy and elevated surfaces. Best timing for this will
be between 02z/9pm Thursday through 10z/5am Friday morning.
Snow amounts continue to remain highly uncertain in this event,
however, thinking that raw model output remains way too high in this
scenario. Several limiting factors remain at work, including: (1.)
Persistent above freezing surface temperatures that will help
temper/melt the majority of the snowfall. The exception to the rule
may be areas closer to the VA/NC border, however, these are the
areas that will likely see far less QPF throughout the event (2.)
10:1 snow to liquid ratios are not likely in an event like this,
expect closer to 3:1 at precipitation changeover onset, 5:1 toward
the middle of the event, and perhaps ending in vicinity of 10:1 if
we are lucky. (3.) While the CAA process will be key in the original
changeover, it will also become the ultimate limiting factor through
the frozen ptype portion of the event. Gradual drying is expected to
persistently take place in the lower-third of the boundary layer as
the cold air slides south into the region. This will work to greatly
reduce QPF from northwest to southeast in general. Higher amounts of
QPF in the southeast will remain, however, dewpoints in the middle
30s will keep widespread snowfall accumulations from occurring.
With these limiting factor in mind, have shifted the geographic
location of accumulations a bit further north and west. Amounts are
down a bit also with this run, topping out in the 1 - 2" range in
northeastern Coastal Plain. From there, if you follow the US-64
further west, expect accumulations in vicinity of an inch with
lesser amounts as you progress west of US-1 and south of US-64.
Alterations and fine tuning to the accumulation forecast is highly
likely to continue with sequential model runs and forecast packages,
but still finding it highly doubtful that significant snowfall
amounts will occur across central NC. For the snow lovers out there,
the canary in the mine that could indicate the potential for higher
amounts lies in surface dewpoints Thursday afternoon. If dewpoints
become significantly cooler, a possibility advertised by the
recently cold-initialized NAM, wet-bulb cooling processes may
promote an earlier changeover and perhaps higher amounts somewhere
across central NC assuming the moisture is available.
4:10am
What has changed: Reduced snowfall amounts a bit across the
southeastern zones and focused the peak accumulation amounts (1-2")
a bit further north and west to better coincide with latest model
trends. Our forecast remains much lower than most raw forecast
guidance due to numerous limiting factors, including; (1) Warm
surface temperatures, (2) Self-limiting CAA processes, and (3) Low
rain to snow liquid ratios. Impacts should remain rather minimal,
however, a few slick spots may form on area roadways during the
overnight hours Thursday into early Friday morning, primarily along
and east of US-1. If the forecast holds true, a Winter Storm
Advisory issuance may be needed in future forecast packages (Our
criteria is areas that are expecting to see 1" of snow or greater in
a 12 hour period).
Full Discussion: The surface front will settle in a nearly west to
east orientation in vicinity of the SC/GA border, with ongoing
surface CAA processes occurring north of the boundary. The front is
progged to buckle back north as a series of weak disturbances begin
to develop during the day Thursday, with strong coastal cyclogenesis
beginning to initialize off the SC coast as early as Thursday
evening. Around the same time, strong Canadian surface high pressure
will continue to settle southeast through the central Corn Belt
states, extending east through the Great Lakes and northeastern US
states before beginning to south along the lee side of the
Appalachians.
Location and strength of the developing coastal low/trough will be
key to the forecast going forward, as a stronger and closer to shore
area of low pressure (NAM) will help to promote more rapid cold air
entrainment from the north. Forecast models continue to trend
increasingly more wet during the daytime hours Thursday, especially
from the US-64 corridor south during the afternoon and late evening
hours. Around this time, a 925mb cold front (marking the leading
edge of the cold nose) will extend south into the area. This will
aid, at least initially, in the ability for wet bulb/evaporative
cooling processes to promote a light rain to snow changeover to
occur in vicinity of the VA/NC border mid to late afternoon. From
there, a gradual changeover to a rain/snow mix to eventually all
snow will occur. At first, accumulations will remain little to none
with more of a "white rain" type scenario unfolding. Eventually,
especially areas east of US-1 where rain rates will remain higher
for longer, the wet-bulb effect will help to cool surface
temperatures below freezing, helping to promote accumulations
primarily on grassy and elevated surfaces. Best timing for this will
be between 02z/9pm Thursday through 10z/5am Friday morning.
Snow amounts continue to remain highly uncertain in this event,
however, thinking that raw model output remains way too high in this
scenario. Several limiting factors remain at work, including: (1.)
Persistent above freezing surface temperatures that will help
temper/melt the majority of the snowfall. The exception to the rule
may be areas closer to the VA/NC border, however, these are the
areas that will likely see far less QPF throughout the event (2.)
10:1 snow to liquid ratios are not likely in an event like this,
expect closer to 3:1 at precipitation changeover onset, 5:1 toward
the middle of the event, and perhaps ending in vicinity of 10:1 if
we are lucky. (3.) While the CAA process will be key in the original
changeover, it will also become the ultimate limiting factor through
the frozen ptype portion of the event. Gradual drying is expected to
persistently take place in the lower-third of the boundary layer as
the cold air slides south into the region. This will work to greatly
reduce QPF from northwest to southeast in general. Higher amounts of
QPF in the southeast will remain, however, dewpoints in the middle
30s will keep widespread snowfall accumulations from occurring.
With these limiting factor in mind, have shifted the geographic
location of accumulations a bit further north and west. Amounts are
down a bit also with this run, topping out in the 1 - 2" range in
northeastern Coastal Plain. From there, if you follow the US-64
further west, expect accumulations in vicinity of an inch with
lesser amounts as you progress west of US-1 and south of US-64.
Alterations and fine tuning to the accumulation forecast is highly
likely to continue with sequential model runs and forecast packages,
but still finding it highly doubtful that significant snowfall
amounts will occur across central NC. For the snow lovers out there,
the canary in the mine that could indicate the potential for higher
amounts lies in surface dewpoints Thursday afternoon. If dewpoints
become significantly cooler, a possibility advertised by the
recently cold-initialized NAM, wet-bulb cooling processes may
promote an earlier changeover and perhaps higher amounts somewhere
across central NC assuming the moisture is available.
Forevertothee
Member
GSP
While guidance continues to disagree on total QPF with this event
(as has been the trend the past few days), the general consensus
continues for cyclogenesis along the front somewhere off the GA/FL
Coast on Thursday, as the parent sfc low then moves just off the
Carolina coast into Thursday afternoon. With upper dynamic support,
along with moisture infiltrating in across the Gulf states and
Southeast, precipitation is expected across the area Thursday,
tapering off into Thursday night as the system pulls away. Expect
precip to be ongoing at the beginning of the fcst period, with
mainly snow across the mountains, and all rain across northeast GA
and the Upstate given temperatures around daybreak in the lower 40s.
Temperatures are expected to decrease throughout the rest of the
day, and as this occurs, precipitation is expected to expand across
the area. Per latest fcst soundings, the NAM continues to be the
outlier suggesting a small window of wintry mix around midday as
precipitation transitions from rain to snow. Meanwhile, both the GFS
and ECMWF suggest a clean cut rain to snow transition. Thus, have
kept with this trend into the early afternoon hours, though wet bulb
temps suggest the transition could occur sooner, around noon,
especially around the CLT Metro area. Something to keep a close eye
on. Throughout the afternoon hours, the rain/snow line will shift
southward into the Upstate as temperatures continue to drop.
However, as this transition occurs into the evening, QPF will
decrease significantly as well. Thus, with this fcst package, the
Upstate and portions of northeast GA will struggle to see much in
the way of snow accumulation, while across across the NW Piedmont
could see a dusting to near an inch, and slightly higher amounts
across the mountains, especially across the higher elevations.
Expect all precipitation to dissipate overnight. Any changes in the
next guidance could alter snow accumulations, and could warrant the
issuance of a Winter Weather Advisory.
Sent from my iPhone using Tapatalk
While guidance continues to disagree on total QPF with this event
(as has been the trend the past few days), the general consensus
continues for cyclogenesis along the front somewhere off the GA/FL
Coast on Thursday, as the parent sfc low then moves just off the
Carolina coast into Thursday afternoon. With upper dynamic support,
along with moisture infiltrating in across the Gulf states and
Southeast, precipitation is expected across the area Thursday,
tapering off into Thursday night as the system pulls away. Expect
precip to be ongoing at the beginning of the fcst period, with
mainly snow across the mountains, and all rain across northeast GA
and the Upstate given temperatures around daybreak in the lower 40s.
Temperatures are expected to decrease throughout the rest of the
day, and as this occurs, precipitation is expected to expand across
the area. Per latest fcst soundings, the NAM continues to be the
outlier suggesting a small window of wintry mix around midday as
precipitation transitions from rain to snow. Meanwhile, both the GFS
and ECMWF suggest a clean cut rain to snow transition. Thus, have
kept with this trend into the early afternoon hours, though wet bulb
temps suggest the transition could occur sooner, around noon,
especially around the CLT Metro area. Something to keep a close eye
on. Throughout the afternoon hours, the rain/snow line will shift
southward into the Upstate as temperatures continue to drop.
However, as this transition occurs into the evening, QPF will
decrease significantly as well. Thus, with this fcst package, the
Upstate and portions of northeast GA will struggle to see much in
the way of snow accumulation, while across across the NW Piedmont
could see a dusting to near an inch, and slightly higher amounts
across the mountains, especially across the higher elevations.
Expect all precipitation to dissipate overnight. Any changes in the
next guidance could alter snow accumulations, and could warrant the
issuance of a Winter Weather Advisory.
Sent from my iPhone using Tapatalk
packfan98
Moderator
6z gfs looks similar to 0z. Maybe a little less precip on the nw edge of the system.
Six Mile Wx
Member
For western upstate SC we really need the cold sooner or moisture later for anything measurable. The sooner we transition to snow the better. Anything before the 1:00pm -4:00pm transition time is a bonus. Any transfer later than 4:00pm looks to be an issue with moisture unless the bulk of the precip slows. An early transfer to all snow, say 9:00-10am is a BIG DOG!
packfan98
Moderator
Here's the HIGH END map from the NWS for NC. If you were to press me on a forecast map, I'd trim this by about 20-25% and say it would be pretty close to what happens.
NCCWOP9077
Member
Overnight EPS playing catch-up showing 43/50 members with measurable snow and a mean average of 3 inches at the Johnston county airport.
Sent from my SM-J727U using Tapatalk
Sent from my SM-J727U using Tapatalk
Webberweather53
Meteorologist
Here's the HIGH END map from the NWS for NC. If you were to press me on a forecast map, I'd trim this by about 20-25% and say it would be pretty close to what happens.
I think for southern zones it's a smidge overdone, but in the northern coastal plain I think the totals will actually be higher than that.
Webberweather53
Meteorologist
I'm thinking something in the ballpark of 4-8" for the Triad & Triangle areas.
NCCWOP9077
Member
@Webberweather53 how about Wilson and Johnston counties? I know there will be some mixing early on, but maybe some decent snowfall as well.
Sent from my SM-J727U using Tapatalk
Sent from my SM-J727U using Tapatalk
packfan98
Moderator
I'm curious on your thoughts about the NWS discussion. It seemed well written and thought out, but maxed out accumulations at 1-2 inches and no watches being issued. I'm guessing that they will ramp things up this afternoon, but that will be with less than 24 hours lead time.
snowlover91
Member
I have to say I strongly disagree with the NWS discussion on snowfall amounts. I understand they are playing it safe right now but their discussion about temperatures being an issue isn’t valid imo if you look at the best model for thermals in this range, the 3km.
Temps QUICKLY crash to 32 across areas as the moderate to heavy precip moves in on the 3km nam. It is the best model imo with thermal profiles inside 48 hours and the NWS has been burned many times in recent memory by ignoring it.
Once the event really gets going we have a large area of 30-32F temps.
Also regarding precip, the 3km NAM is forecasting close to an inch for the RDU to Wilson corridor and then 1-1.5” of qpf East of there. Assuming the NAM rain/snow line is correct, even an average 5:1 ratio as discussed would yield 4-5” for Raleigh to Wilson and 5-8” east of there. I think they’re making the mistake of looking at globals for thermal profiles, something they’ve done repeatedly over the years while seemingly ignoring the 3km NAM for some unknown reason.
Temps QUICKLY crash to 32 across areas as the moderate to heavy precip moves in on the 3km nam. It is the best model imo with thermal profiles inside 48 hours and the NWS has been burned many times in recent memory by ignoring it.
Once the event really gets going we have a large area of 30-32F temps.
Also regarding precip, the 3km NAM is forecasting close to an inch for the RDU to Wilson corridor and then 1-1.5” of qpf East of there. Assuming the NAM rain/snow line is correct, even an average 5:1 ratio as discussed would yield 4-5” for Raleigh to Wilson and 5-8” east of there. I think they’re making the mistake of looking at globals for thermal profiles, something they’ve done repeatedly over the years while seemingly ignoring the 3km NAM for some unknown reason.
I gotta say personally I was leaning towards those lower amounts too but the RGEM has me rethinking that. I've always liked that model and the 6z run was amazingHere's the HIGH END map from the NWS for NC. If you were to press me on a forecast map, I'd trim this by about 20-25% and say it would be pretty close to what happens.
I see a call map thread has been started which is fine, but it's a good time to tell y'all not to start an obs thread. Staff has discussed it and we request keep storm specific observations in this thread. Thanks
Webberweather53
Meteorologist
Here's my first call map w/ actual totals. I think the northern coastal plain and NE piedmont is going to get clocked.
snowlover91
Member
IMO the model to follow is the 3km NAM and RGEM later today blended together. Everything has been trending to the overall NAM idea. Rgem caved, euro caved, and other models have too in recent days. The NAM and RGEM blend at 12z today should be pretty close to what actually happens. The 6z RGEM was a massive jump towards the NAM too with widespread 4-5” amounts and still snowing at the end of the run on pivotal.