jconsor

While I agree that extratropical (non-MJO) forcing played a significant role in the cold Jan 2025 outcome across the eastern/central US, we need to be careful using VP averaged across an entire month. Subseasonal variability clearly played a role here. In addition, the NCEP reanalysis has some notable biases in VP - it tends to show overly positive VP anomalies near C/S. America and Africa. See Stanfield et. al 2024, A Climatological Analysis of Upper-Level Velocity Potential Using Global Weather Reanalysis, 1959–2020: https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/joc.8659 The vast majority of the cold anomalies in the eastern/central US occurred in the first two-thirds of Jan. From around Jan 23 onward it turned warmer than normal. The Jan 1-10 and Jan 11-20 VP anomalies show -VP anomalies migrating from the central/eastern Pacific and W. Atlantic, to the e. Atlantic and w. Africa. The magnitude is not as high as the negative anomalies that developed from the eastern IO to Maritime Continent in the Jan 21-31 period. However, especially in the Jan 11-20 period I would argue the MJO had a significant role, given the coherent wave of -VP (and corresponding +VP across the c/e. Pacific) that developed.

The Jan 22, 1987 snowstorm you are referring to is one of the main events that got me interested in weather. I grew up on the North Shore of LI (Roslyn) and I recall the forecast being for an inch or two, then quickly changing to rain. Instead snow piled up at 1-2" per hour and there was never a change to plain rain, just a little freezing drizzle at the end. As Ed said, it was traffic chaos. I was in second grade at the time and we were stuck at school for about 6 hours! The plows couldn't keep up with the snow and the hills on the roads in/around Roslyn made things even worse.

Some additional context to the unusual nature of this cold outbreak stratosphere-troposphere wise: https://bsky.app/profile/drahbutler.bsky.social/post/3lekmlyusmc25

Indeed, as we get closer in the guidance is correcting to a stronger high pressure and CAD wedge for the last week of Dec. https://x.com/yconsor/status/1870772344031268877

https://x.com/yconsor/status/1869457576339136768

https://x.com/yconsor/status/1869457576339136768

MJO phases 5 and 6 are far from a guarantee of warmth in the eastern US. https://x.com/commoditywx/status/1866191470287286392 https://x.com/bamwxcom/status/1866093279948763384

Per Eric Webb, the PDO has limited influence and limited usefulness for subseasonal predictability (especially in a winter like this with a strong Indo-W Pacific warm pool). https://x.com/webberweather/status/1864088518886322455 https://x.com/webberweather/status/1864091992386605400 https://x.com/webberweather/status/1864095069449699705 https://x.com/webberweather/status/1864150597710369022 Also interesting comments on PDO's limited usefulness as a teleconnection here: https://x.com/_WxPhil_/status/1864148237403263157

https://x.com/yconsor/status/1850618329804730523

Some short-term drought relief possible this weekend: https://x.com/yconsor/status/1848326240068837640

Thread on important short-term northward/stronger trends noted regarding Beryl, and how this influences her potential TX/MX impacts: https://x.com/yconsor/status/1808853711629746332

@bluewave Nice model bias charts! Can you please provide the link for them? Thanks.

Yes, within 10 days of the upcoming month is the CFS's more skillful phase. But much of that skill hinges on getting the tropical convection/MJO pattern correct. CFS tends to progress the MJO eastward too slowly beyond 7 days. Look how different it is vs. EPS and GEFS even in the 8-14 day period! CFS hangs the -VP anomalies back over far e. Africa and w. IO, whereas GEFS and EPS progress them eastward into the e. IO and Maritime Continent. Looking at longer-range hovmollers, CFS continues to progress the MJO eastward sluggishly, in a manner that to my eyes doesn't make sense. It has the MJO in phases 4 and 5 in late Mar. Meanwhile, the extended GEFS and EPS have the MJO progressing into phases 8 and 1 by late Mar. The implications are very different! The phase 4 and 5 composites largely support the CFS's warm forecast for the second half of Mar. The phase 1 and 8 composites favor a -NAO, western N. America ridging and eastern US troughing, which would suggest normal to colder than normal temps in the second half of Mar. The forecast SSW event in early Mar should have a significant influence in prolonging blocking near Greenland/-NAO in mid to late Mar. I am not sure that this stratospheric warming will behave like a final warming in terms of its influence, given how strong it is forecast to be and how early (EPS indicates the warming peaking on Mar 6-7, which would be the second earliest final warming in the past 40+ years). I agree that Mar 1-10 will feature well above normal temps in the NE US. Mar 11-15 should see a transition period, and in the second half of Mar I would expect slightly below normal temps. The large magnitude of the warm anomalies Mar 1-10 will likely make it tough for the month as a whole to average below normal. But still, I think the CFS forecast for the second half of the month is highly questionable. I would expect the 500 mb/sfc temp pattern for the last half of Mar to be close to a blend of clusters 2 and 3. Cluster 4 (which the CFS forecast is indicating) is close to the prevailing pattern since Dec.

I don't recommend using the HREF snowfall totals verbatim. Its snow-liquid ratios tend to be too high. IMHO, Best to take the probability-matched mean QPF and then convert to snowfall using expected snow-liquid ratios (should be generally 15 to 18-1 in this case).

ENSO Longitude Index in 12th place since 1950 puts us squarely in Modoki territory. See https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL079203 for more details on the ENSO Longitude Index (ELI)

Dec Modoki Ninos are much more likely to have a -NAO than east-based Ninos. Basin-Wide Ninos have a slight tendency toward +NAO. Given Eric Webb's recent tweet about the implications of Dec NAO in Ninos on the pattern during the rest of the winter, will be very interesting to watch the NAO trends as Dec goes on.

There is a decent chance this system becomes a subtropical or tropical cyclone in the next day or two as it heads NE up the Gulf Stream.

Mesovortex off the coast of SE FL is causing impacts similar to a tropical storm there. Widespread gusts to 50-60 mph, locally up to around 75 mph:

Signal for tropical cyclone formation in the W. Caribbean by late next week (and northward movement thereafter) has grown.

While climo is heavily in favor of no TC development from here on out, the pattern suggests we still need to keep a wary eye on the Caribbean next week.

Thanks for starting the thread, Ed. The ECMWF and EPS have been handling Tammy's intensification and track poorly (GEFS has had better handle). Hopefully with the 12Z run the EPS should be a more useful tool, once recon data is ingested.

Thread on the flooding potential:

I know most of our collective attention is on the potential subtropical system off the SE US coast. However, I think the system in the E. Atlantic (currently given a 70% chance of developing by NHC) is worth watching carefully, since down the road in 7-10 days it has a decent opportunity to reach the Caribbean or western Atlantic. This is a sneaky wave which will likely not develop in the next 3-4 days, but instead continue heading west under a strong ridge. We're dealing with two waves that will likely merge here - firstly the one outlooked by NHC that just came off the African coast, and secondly a low-latitude ITCZ wave currently around 40W. The two systems should combine by late Fri-early Sat into a broad wave. This would initially slow development, due to its large size and lack of concentrated vorticity. This system likely will find better upper level conditions (strong diffluence aloft, deeper moisture) once it passes 40W and clears the strong UL trough in the east Atlantic, and especially as it nears 60W and the Leeward/Windward islands. With a much stronger than usual ridge for this time of year over the central Atlantic, there is a decent chance this system makes it into the eastern and northern Caribbean or heads slightly N of the Caribbean to near the Bahamas.

Very helpful summary of the historical context. Thanks, Larry! I think your summary shows that given a few days (even as little as 36-48 hours) over water off the SE US coast, a system of non-tropical origins with a decent upper-level wind setup can become a hurricane. Of course, the majority of such systems remain at tropical/subtropical storm strength. FYI today's 0Z run of the NASA GEOS-5 model is showing what looks to be a high-end TS or cat 1 hurricane, certainly a more consolidated system than the global models (GFS, GEM, ECMWF) are indicating. https://fluid.nccs.nasa.gov/wxmaps I believe the NASA GEOS-5 model is run at 7 km resolution.

Regardless of whether the system off the SE US coast becomes subtropical, there is a substantial risk of heavy rainfall and flooding from eastern NC to the eastern mid-Atlantic and Long Island.