A cold airmass over the Great Lakes states and northeast that allowed an unprecedentedly early snowfall over central Pennsylvania (see precipitation totals ending at 1200 UTC on 16 October here) is also supporting the development of Lake-effect precipitation downwind of Lake Michigan. In the loop above (click the loop to enlarge), pay attention to the cumuliform cloud development over the relatively warm waters of Lake Michigan, and note how the straight lines of clouds move south-southwestward towards the lake counties of Wisconsin and Illinois. These cumuliform clouds contain showers as noted in the radar imagery here. The plot of surface data (below) at 1800 UTC shows a prevailing northerly or north-northeasterly flow at the surface, consistent with the motion of the lake-effect clouds. Note also the plots from moored buoys over the Lakes (plotted in red) that show temperatures in the low 50s (11-12 Celsius) over Lake Michigan and mid 40s (7-8 Celsius) over cooler Lake Superior.

The 850-hPa temperatures observed by radiosondes over the midwest at 1200 UTC on 16 October (here) show temperatures cooler than -5 Celsius over Lake Michigan. In general, a temperature difference of at least 13 C between 850 hPa and the Lake Surface is looked for when forecasting lake-effect precipitation. Observations in the surface plot certainly support a temperature difference exceeding 13 C between the lake surface and 850 hPa. Estimates of lake surface temperature from satellite have been hampered lately by persistent cloudiness over the Great Lakes basin. An MODIS estimate from 1800 UTC today, however, located here, does show temperatures in the 50s over Lake Michigan, and somewhat cooler waters over Lake Superior. This cooler lake temperatures in Lake Superior may explain in part the lack of lake-effect clouds just downwind of Lake Superior over the eastern part of the upper Peninsula (It is likely that other forcings may be suppressing cloud formation there as well; note that at the beginning of the visible imagery loop, cloud streets do extend from Lake Superior into the upper Penisula of Michigan just to the east of Marquette; these cloud streets dissipate with time, suggesting strong subsidence in the region).

Development of lake-effect clouds in mid-October is a reminder of what is to come in the near future as cooler and cooler airmasses develop over Canada and move southward over the still-warm Great Lakes.

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Snow is widespread today from central Pennsylvania northward into central New York. Accumulating snow only rarely falls this early in the season in that part of the country, and at many stations this is the earliest measurable snow on record. The above image is an enhanced 11-micron image from GOES-12 with surface observations (in white) and buoy observations (in black) superimposed. (Click image to enlarge)

One reason for snow’s rarity can be viewed in the sea surface temperatures plotted in black — ocean surface temperatures are still in the low 60s off the coast of New Jersey. That source of relative warmth can have a powerful effect if the wind is from the east, and it can be fatal to a snowfall.

Much of the moisture from this storm, however, is not coming from the east, but from the west and southwest, as suggested in this plot of total precipitable water — plotted as a percentage of normal — derived from AMSU and SSM/I instruments on the NOAA polar orbiters. (See image here; note the axis of high percentages along the spine of the Appalachians).

A vital feature for the production of snow is the presence of ice crystals within a mixed-phase cloud. When that happens, the Bergeron-Findeisen process allows ice crystals to grow at the expense of water droplets, and these ice crystals can then fall towards the surface, either maintaining their integrity as snow all the way down or melting to rain drops. If ice crystals are not present, cloud droplets will grow via Collision and Coalescence, and a drizzle or light rain is more likely. Cloud types derived from different channels on MODIS (image here, valid at 1534 UTC on 15 October) and from AVHRR (image here, valid at 1721 UTC 15 October). Both images suggest that the presence of ice crystals for the seeder-feeder mechanism might be limited if winds at cloud level are westerly. Should that happen, the snow of this afternoon would become light rain or drizzle tonight. MODIS cloud phase from 1710 UTC of 15 October, however, which has a swath farther to the west, does show more ice clouds upstream of Pennsylvania and New York, an observation that suggests snow may continue, if surface and near-surface temperatures remain cool enough.

(Updated, 16 October: Some snow totals as of 1200 UTC 16 October: 4.7″ in State College, 3.2″ in Philipsburg, 2.4″ in Altoona, 5.9″ in Wellsboro)

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Columbus Day is the Anniversary of a truly historic storm in the Pacific Northwest. On October 12, 1962, one of the most intense storms on record caused wind gusts exceeding hurricane force over a broad swath of coastal Washington and Oregon. Now, 47 years later, a weaker but still potent storm is poised to move onshore.

Water vapor imagery (above) shows the characteristic signal of a strong jet, with a dark band, indicating sinking and drying (and warmer brightness temperatures because the satellite sensor is seeing farther down into the atmosphere), on the poleward side of the strongest winds. Aircraft wind observations (plotted in red) show numerous observations of wind speeds exceeding 150 knots, and the GFS analysis 6-hr forecast valid at 1200 UTC this morning (the time of the water vapor image) shows a 160+ knot jet on the 345 K isentropic surface.

The big storm of 1962 could be easily linked to a tropical cyclone that moved north just west of the Dateline. Is the jet now present in the Pacific linked to Typhoon Melor, which storm was off the coast of Japan last week? This animation of enhanced 11-micron imagery over the Pacific Ocean suggests that it is. Certainly the moisture from the tropical system is part of the jet; careful inspection of the imagery suggests that the mid-level vorticity from Melor is also involved in this jet.

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MTSAT IR image

Tropical Systems can occasionally be accompanied by Predecessor Rainfall Events (PREs). This band of heavy rainfall is not associated with the spiral bands of the tropical system, but rather with an interaction with a mid-latitude jet that exists Poleward of the tropical feature. Some notable PRE-producing tropical systems in the United States include Hurricane Ike from 2008 and Hurricane Floyd from 1994. Accumulated rainfall patterns from Ike and from Floyd show very heavy rains far removed from the landfall; in Ike’s case, over northwestern Indiana, and in Floyd’s case over Connecticut and New York. In both cases, PREs have been identified as a likely rain producer.

Typhoon Melor has been approaching the northwestern corner of the tropical Pacific Ocean over the past several days. At the same time, a ribbon of moist air, as denoted by high Precipitable Water values diagnosed from MIMIC, extends southwestward from Japan towards the straight of Luzon (Note also in the Precipitable Water loop the presence of former Typhoon — now Tropical Storm Parma meandering within the straight of Luzon as well.

The 11-micron window channel imagery show a general blossoming of cold cloud tops in and around the ribbon air with high precipitable water as typhoon Melor approaches (Link here). Indeed, the last image in the loop, shown above in this post, bears a resemblance to the enhanced 11-micron GOES-8 image from landfall of Floyd.

Is there evidence of a jet poleward of Melor that would support the development of the PRE? Consider the enhanced water vapor image from MTSAT below. The large gradient in brightness temperature — very warm values in and around Korea and the South China Sea northeastward to the Sea of Japan, and very cold temperatures to the east, suggest the presence of a strong jet. Plots of 300-hPa wind speeds confirm that; note the speeds exceeding 150 knots at Sapporo and at Nakashibetsu on the island of Hokkaido! The position of this jet is such that the left entrance region is supporting upward motion to help support heavy rain in a very moisture-rich atmosphere. (The GFS 00-h analysis at 1200 UTC 6 October shows this jet extending far out into the Pacific Ocean).

MTSAT water vapor image + 300 hPa rawinsonde wind speeds

MTSAT atmospheric motion vectors or “satellite winds”  (calculated by tracking satellite image features on 3 consecutive images) actually showed that wind speeds along the jet stream axis over the western Pacific Ocean were as high as 211 knots at the 218 mb level (below).

MTSAT water vapor image + MTSAT winds

(Added: TRMM measurements of rainfall with Melor are here).

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