Welcome to the weather blog - a regular Monday through Friday discussion of the weather! While the discussions usually will only come on days I'm working, I'll issue special updates when the weather warrants. I will always post to let you know when no discussion is expected if I'm away on vacation, etc. - if no update is here and no info is available, that likely means the server has temporarily gone on the fritz and I will update as soon as technically possible. You'll find a quick weather synopsis linked to the daily forecast at the top of the page, a general non-technical weather summary below, and when available (most days) a detailed technical meteorological discussion will follow by mid-afternoon. My email is contact@mattnoyes.net. This blog is for you, so I hope you enjoy it! -Matt Noyes
General Weather Summary:
The jet stream winds aloft - the fast river of air high in the sky that steers our storms and acts as a thermostat for the atmosphere, separating warm air to the south from cooler and drier air to the north - continue to flow fast across New England. Not only does this keep us in a progressive weather pattern that swings new disturbances through frequently, but it also places New England on a battlezone between deep summer warmth and humidity brewing to our south, and shots of cooler and dry Canadian air that continue to penetrate south into the Northeastern United States.
When these two airmasses clash, clouds and precipitation is often the result, but for the time-being New England remains in a break between systems, and in a moderating dry Canadian airmass. The result was a cool start Tuesday morning with areas of valley fog that burned off to a gorgeous blend of sun and clouds. Nonetheless, a reinforcing shot of cool and dry air is settling southward through Quebec and Ontario, and the leading edge to this push of new air is represented by a cold front. Though this front will bring a few showers and thunderstorms to the North Country by later Tuesday afternoon and early Tuesday evening, the combination of sunshine and a steady west wind ahead of the cool front will usher in a somewhat warmer airmass. With the west winds sloping down off the mountains and hills of New England, this should be an added impetus to push temperatures well into the 80s for most communities. Keep in mind that while a few late-day storms are possible in the far North, most of us will stay dry and enjoy a wonderful day. The cold front slicing through Northern New England late Tuesday will slow as it crosses Southern New England Tuesday night, nearly stalling along the South Coast of New England by Wednesday.
Though there will be few effects from this front Tuesday night with partly cloudy skies and perhaps just a slight wind shift, the front is likely to have two effects on Wednesday: 1) A noticeable temperature difference will be present from north (60s) to south (lower 80s) and, 2) a few showers or a thunderstorm will still be possible in Southern New England on Wednesday, especially during the afternoon. Nonetheless, this front represents the leading edge to another shot of dry and cool Canadian air, and the center of this air - defined on the weather map by an area of high pressure - will settle just east of New England on Thursday. The clockwise flow of air around this fair weather center will be important, as it will favor an "onshore" flow - that is, air blowing in off of the Atlantic. This will keep temperatures from climbing too high on Thursday afternoon, especially when combined with high altitude clouds that may dim the sun through the day.
Eventually, a clash will set up in the atmosphere over New England. This clash will come by the end of the week, as the cool and dry Canadian air over New England will be invaded by a strong surge of warmth and moisture pushing northeast out of the Gulf of Mexico and across the Ohio Valley. The collision of airmasses is likely to create at least a period of showers and perhaps a period of rain sometime Thursday night and into early Friday. While conditions will gradually become more humid on Friday, especially after the passage of early rains and the gradual emergence of breaks of sun, it's more likely that Saturday will find Southern New England emerging into deep summer warmth for a brief time - with humid conditions and highs into the 80s - while Northern New England is likely to remain mostly cloudy and much cooler. Though showers and thunder will be likely Saturday afternoon on the battlezone between these two airmasses laid across New England, a period of heavier rain will be possible later Sunday into early Monday with the approach of yet another storm center and attendant cold front.
In the tropics, the strong wave we discussed yesterday that was pushing off the African coast has developed into Tropical Depression #4 and will likely be named Tropical Storm Debby by the end of Tuesday. This storm should move west-northwest over the Central Atlantic, strengthening as it does so, and will continue to be watched for several days, though is nearly two weeks from any possible threat to the United States, if it should survive the trip. A new cluster of thunderstorms bears watching approaching the Windward Islands, and will be entering an area more favorable for storm development over the next 48 hours as it crosses the Windward and Leeward Islands. Finally, the cluster of thunderstorms northeast of the coast of Nicaragua, located in the Western Caribbean Sea between Cuba and Nicaragua still has not organized yet, and conditions still remain somwhat unfavorable for development.
Enjoy your Tuesday!
Matt
Matt's Technical Meteorological Discussion: Updated Tuesday, August 22 at 2:10 PM
Tuesday afternoon/eve shortwave is driving cold front southward thru Quebec en route to North Country where well developed Cu line ahead of front has been migrating east out of Nrn NY. Diurnal instabililty will help to expand activity east with CAPE over 1000 J/kg and LIs to -4. Even with this sfc based instability, there is plenty of dry air aloft and that will fight development of more widespread convection, tho line segments certainly are a possibility along the low level convergence zone. Though the mid level forcing does push south into Srn NewEng Tue eve, and this may carry a few quickly weakening showers with it, instability will be waning by that point and dry air is abundant below and above max lift at 700 mb and this won't favor survival of the convection.
Result should be a quiet night with clouds gradually shrinking and front with weak wind shift settling slowly southward. This should create a decent difference in overnight min temps from widespread 50s and some 40s north to 60s south.
With such dry ambient atmosphere, sunshine will quickly work on lingering warm air in Srn NewEng Wed AM to boost temps to around 80 while low level cold advection will hold temps down across the North. We've seen no post-frontal precip with this front in Canada and I don't see much reason to expect any tomorrow, so key to landing area of precip threat is landing where front stops, and the trend has been farther north on especially the E side. 12Z cycle of NAM tries to develop wave along frontal boundary in Ern MA, and given superior handling of low level airmasses with high resolution, it makes sense to keep at least chc shra/tsra in for most of Srn NewEng Wed afternoon, tho not expecting action in Western MA, Ern areas will exceed 1500 J/kg CAPE.
Passage of Wed shortwave shunts front southward and Thu bubble high will not only bring cool Canadian air but also will turn winds onshore many locales which will contribute to cool afternoon for all. One shortwave will bring warm advection overhead late Thu and Thu Ngt which should produce precip as isentropic lift occurs over cool dome, especially Thu eve and night. With the general pattern of synoptic warm advection and a parade of shortwaves, timing of rainfall will be dependent upon timing of shortwaves, which always becomes clearer the closer we draw to the forecast period in a fast flow, and that timing looks like a break to start Friday but another shield of precip later and into the night. Still, of course, subject to adjustment dependent on how the timing shakes out.
Overall, though, keep in mind that this is part of a transition to warmer air and the process allowing for the repetitive rains is in fact the advancement of the deep summer warmth and humidity from the southwest. Eventually, this warm and moist assault will erode the southern edge of the cool and dry Canadian airmass and Southern NewEng should be back into soupy air on Sat, tho given confluent flow over Nova Scotia at 500 mb thanks to the broad trof over the Canadian maritimes, I have trouble believing we can blast this warmth into Northern NewEng, where overrunning showers are likely to continue periodically with plenty of clouds and resultantly cool temps. Another in the parade of shortwaves should touch off thunder in Southern NewEng later Sat. By Sun into Mon, extremely strong bowling-ball upper low moves E from Upper Great Lakes and thru NewEng, and this sets the stage for a heavy rainfall event in that time frame. Ensemble means have remained in excellent agreement on heavy amounts with this event, tho timing has trended slightly slower, largely owing to strength and amplification of the incoming system.
Tropics: The tropics have been heating up recently, as well, and while I've been less attendant to the tropics in this technical discussion this year contrasted to last, it's largely because there hasn't been a great deal to speak of that I felt was worth putting so much time into. Thinigs are changing quickly in the Tropical Atlantic, though, and I think the East Coast of the United States will be in a precarious position for the coming weeks - especially the Southeast and Mid-Atlantic coastlines, but NewEng is always just one good shortwave interaction away. Let's set the stage first, which is that we've seen very little action thus far in the Atlantic, owing to a combination of factors led by an over-abundance of dry air in the Western Atlantic and the Caribbean, leaving deep moisture confined to the intertropical convergence zone, where storm development is very difficult to achieve. There also has likely been at least a week feedback mechanism with the Eastern Equitorial Pacific, which, as mentioned in this discussion a few weeks ago was showing El Nino indications with a warm tongue of sea-surface temperatures extending west off the coast of South America and indicating a weakening in the Equatorial Pacific trade winds. This anomaly, though present, has weakened since that time. Additionally - and this is not a scientifically tried and tested fact - but the negative PNA (Pacific North America Pattern) of late has favored higher pressures over the Caribbean, Western Atlantic and Southeastern US, and this of course is not conducive to tropical development, but the PNA is forecasted to reverse in the coming week and last as positive into the beginning of September. Again, no statistical correlation here, just something worth keeping an eye on. Sea surface temps are doing well across most of the Atlantic with 1-3 Celsius anomalies except in the Eastern half of the Atlantic, where temps are slightly below normal. The overall pattern in the tropics feaures the semi-permanent Central Atlantic ridge though a few mid-level troughs are found on its periphery - one near 30N/60W, and the other near 18N/38W. Of course, given the location of TS4/Debby just southwest of the Cape Verde Islands, it is the latter that is important in the forecast of current tropical activity. This mid-level low is not reflected in the upper level wind structure, but is introducing a definite southwest shear ahead of TS4, and the models are unanimously keying in on this SW shear to push the cyclone on a northwest course consistently. I'm not sold on this for a few reasons, aside from simply the fact that the Atlantic ridge is located to the north. Part of my other problems with this is that the mean layer wind flow from the mid-levels to the surface...essential in steering a thus far shallow storm like this one...are from northeast to southwest, then virtually due west around the Atlantic ridge. This knowledge, combined with analysis of water vapor satellite imagery, makes it hard for me to bite into the guidance idea of carrying the storm blindly NW. Rather, it's entirely possible that the storm continues WNW for 24-36 hours, encountering the envelope of little shear that lay just to its west as of this writing and strengthening steadily until encountering the SW shear, which rather than alter course significantly would instead shear the top of the storm, disrupting outflow, weakening the storm, but not turning it far off of a WNW heading. What this means for the long-term future of the storm is questionable, especially since the mid-level trof is forecasted to only slowly decay while moving west - in other words, how long could the cyclone survive continued shear? Eventually, the upper low will interact and decay with the former trough currently to its west, but that will take days to occur and SW shear will remain ahead of the cyclone for several days, which is a long and hard battle for a cyclone to survive. So, my take on the cylone is that she has a long and unlikely battle to win, but if I've misjudged the motion of the low level center and indeed the SW shear pushes her more NW, she will ride north of the shear and therefore have a more fruitful existence for the time-being, though she'll be more subject to shortwave influence later down the line.
This isn't the end of fun in the tropics, however, as another wave moving across the Windward Islands also holds promise. This wave doesn't look tremendously well organized at present, but it's moving into an area of low shear and a general westerly steering flow. While TD4 takes the show for now and keeps the attention of many, this wave has the potential to make far bigger "waves" in the meteorological world if it can capitalize on the increasingly favorable atmospheric conditions. Ahead of this wave, convection has been limited due to negative 500 mb theta-e anomalies, but a lead tropical wave moving into the region should help to modify the airmass while shear continues to decrease and this all should prime the region for the wave currently approaching the Windwards. The big challenge should this thing develop will be to track...with the FSU MM5 moving the system due north as it interacts with the upper trofs mentioned earlier, the Canadian slams the Yucatan with a strong hurricane and the GFS lay in the middle with what is a more reasonable solution. I'm not sold on the northward MM5 track given that I think the separation will be sufficient from the decaying upper trof to avoid pulling the wave north, and the westerly steering flow is strong and forecasted to weaken but remain moderate. This would lead me to believe a more WNW track for at least the time-being.
Finally, a strong wave over Central Africa will emerge off the African coastline a la TD4 in a few days time, but this time there will be no upper trof awaiting it in its path, and the potential for growth - and especially sustained growth - will be better than it has been/looks to be for TD4.
All the while, the Ensembles are showing an eastward shift in the longwave trough over the next 10 days as the barrelling shortwave on Sunday/Monday that brings our heavy rain helps to nudge the trof east. This does create a weakness in the trough off the East Coast during that time period, but with the trof axis so far east it's unlikely we'd have much to worry about in the upcoming 10 day period here in NewEng. Thereafter, however, new shortwave energy begins digging into the midwest, which allows for longwave trough retrogression in the northern stream, while the Atlantic ridge expands westward and rebuilds toward the SE US coastline, forcing the ridge weakness closer to the coastline. The bottom line of this pattern evolution is that just as the tropics heat up, the East Coast is left in a precarious position for the start of September...
That's all for today.
Matt
