From Discussion 25:

"EMILY HAS REMAINED ESSENTIALLY STEADY-STATE DURING THE PAST FEW HOURS AS A HIGH-END CATEGORY FOUR HURRICANE. HOWEVER... AN AIRCRAFT RECON FIX AT 2340Z DID INDICATE THE CENTRAL PRESSURE HAD FALLEN 8 MB SINCE 18Z TO 929 MB... AND A MORE RECENT FIX MEASURED 931 MB.
THE MAXIMUM 700 MB FLIGHT LEVEL WIND IN THE NORTHEASTERN QUADRANT WAS 149 KT... WHICH SUPPORTS MAINTAINING THE INTENSITY AT 135 KT. DVORAK T-NUMBERS WERE 6.5 FROM SAB AND AFWA AND 7.0 FROM TAFB... SO THERE IS NOT MUCH BASIS ON WHICH TO UPGRADE TO A CATEGORY FIVE AT THIS TIME. SINCE BOTH THE OCEANIC AND ATMOSPHERIC ENVIRONMENTS IN THE WESTERN CARIBBEAN APPEAR TO POSE NOTHING OBVIOUS TO WEAKEN EMILY... INTERNAL STRUCTURAL CHANGES WILL LIKELY DETERMINE INTENSITY CHANGES BEFORE EMILY PASSED OVER OR NEAR THE COAST OF THE YUCATAN PENINSULA."

Makes a lot of sense. Also looks like NHC knew they'd have to provide detailed logic as to why not upgrade to a Cat 5.

Also notice they are now mentioning the possibility Emily may not hit Yucatan peninsula at all.